2
State of Smallpox Medical Countermeasures Readiness
“The eradication of smallpox occurred prior to the development of the majority of modern virological and molecular biological techniques. Therefore, there is a considerable amount that is not understood regarding how this solely human pathogen interacts with its host.”
John Connor, in presentation to the committee
January 12, 2024, in reference to Olson and Shchelkunov (2003)
A variety of medical countermeasures (MCMs) have been developed to detect (diagnostics), prevent (vaccines), and treat (biological agents and antivirals) smallpox disease and transmission. This section summarizes the key characteristics of these assets—focusing on gaps and future opportunities, including emerging technologies. One fundamental lesson learned from COVID-19 and mpox emergencies is that research and development for new smallpox MCMs must not only consider the characteristics of the “product” but also must consider the ability to deploy at scale and ensure equitable access.
DIAGNOSTICS, DETECTION, AND SURVEILLANCE
The ability to rapidly detect and diagnose a potential case of smallpox is central to all containment strategies. In the United States, the primary strategy for containment involves vaccination coupled with surveillance (Henderson and Klepac, 2013; Petersen et al., 2015). Outside of environmental detection systems for smallpox, which operate in limited capacity, the early identification of a smallpox case is presumed likely to occur based on clinical suspicion of smallpox or a non-variola orthopoxvirus. Clinical suspicion of
smallpox would be followed by testing for variola virus, whereas continued clinical suspicion of a non-variola orthopoxvirus for which non-variola orthopoxvirus testing is negative would require follow-on testing for the variola virus itself. In the United States, testing would be conducted by a U.S. Centers for Disease Control and Prevention (CDC) Laboratory Response Network (LRN) laboratory, with CDC conducting confirmatory testing on any positive samples with further confirmatory testing at CDC for any initially variola virus–positive results (CDC, 2017b). As there are few practicing clinicians in the United States who have ever seen a person with smallpox, delays in diagnosis based on clinical suspicion are likely.
Testing Options and Utility
U.S. diagnostic testing options to confirm smallpox rely on a variola-specific assay approved by the U.S. Food and Drug Administration (FDA) in 2017. The FDA-approved Variola Virus Nucleic Acid-Based Detection Assay is indicated for “individuals presenting with pustular or vesicular rash illness or other signs and symptoms of Variola virus infection” (FDA, 2024b). This is the only variola-specific assay available in the United States outside of CDC. This test is available at some LRN laboratories as part of a clinical testing algorithm, with positive results requiring confirmatory testing at CDC via single-gene PCR (polymerase chain reaction) testing and subsequent confirmation via additional genomic testing (Hutchins et al., 2008).
Electron microscopy (EM) or cell culture may additionally be used to provide evidence of an orthopoxvirus or indicate infectivity, respectively, however these techniques alone are not diagnostic for smallpox (CDC, 2016, 2017b). However, negative-stain EM is less sensitive than PCR, there are few laboratories with expertise in this technique, and its use may be limited to additional corroborative testing. Tissue and pathology sample evaluation with immunohistochemical staining is another diagnostic option.
Gaps and System Pain Points
Concerns with Clinical Recognition Capability
The identification of smallpox cases requires clinical recognition. While clinician awareness of orthopoxvirus infections has increased in the United States due to the multi-country mpox outbreak, the rise of mpox could also obscure an outbreak of smallpox or another novel orthopoxvirus. A recent report describing the first fatal case of Alaskapox infection showed that over 6 weeks had passed between the patient’s first reported symptoms and a laboratory confirmation of Alaskapoxvirus (Rogers et al., 2024). Additionally, due to the low positive predictive value of smallpox diagnostics
in the absence of known disease,1 the identification of suspected and probable smallpox cases relies on a patient first meeting a clinical case definition prior to testing (CDC, 2017a). These factors could hamper immediate identification of a smallpox outbreak, especially if case definitions are not appropriate or the initial cases present in an atypical fashion. For example, case definitions used in early days of the COVID-19 pandemic were overly strict by emphasizing travel to Wuhan, China (Suthar et al., 2022). Historically, early waves of the smallpox outbreak in ex-Yugoslavia in 1972 where the index case had an atypical presentation without scarring (Ristanovic et al., 2016).
During the 2022 mpox outbreak, once CDC and FDA began supporting orthopoxvirus testing in five commercial laboratory companies, clinicians preferred to use these laboratories over the public health laboratories of the LRN due to more streamlined patient data requirements and perhaps the perception of a more rapid availability of actionable results initially made possible by higher throughput testing capabilities at commercial laboratories (APHL, 2023). The addition of commercial laboratories also increased testing capacity significantly (CIDRAP, 2022). Better understanding of data needs and decision-making and distribution timelines could have implications for laboratory planning for an evolving smallpox outbreak, especially around clinician education, the distribution and ubiquity of laboratory-based tests, and, critically for the questions posed to this committee, the level of investment by the Biomedical Advanced Research and Development Authority (BARDA) in point-of-care diagnostics for the U.S. Strategic National Stockpile (SNS) and for routine use.
Limited Availability of Diagnostic Assays
Diagnostic assays for variola are available only at select LRN laboratories, largely due to biosafety considerations, and only as part of a clinical testing algorithm. The approach to only test at LRNs with confirmation at CDC may affect testing turnaround times during a larger outbreak. The current algorithm was designed for the detection of initial cases in the absence of endemic disease, but planning for laboratory capacity should include strategies for both immediate response and long-term scenarios where greater demand for testing arises. Greater availability of reliable laboratory-based tests for smallpox may enhance the nation’s ability to rapidly respond to a smallpox outbreak, although easier access to testing would also presumably increase the frequency of false positive tests, making effective clinician education even more important. Additionally, expanding the number
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1 For rare diseases, positive tests are more likely to be wrong than when the disease is commonly occurring.
of sites that can conduct both variola-specific and orthopoxvirus diagnostic testing would require concurrent improvements to laboratory biosafety and management of potential associated biosecurity concerns.
Lack of FDA-Cleared Serological Assays
FDA-cleared serological assays for smallpox have not been developed, and protein-based tests are still being explored. Lateral-flow assay tests so far do not offer high enough sensitivity to reliably detect orthopoxviruses and need further confirmation against clinical specimens (Ulaeto et al., 2022).
Future Opportunities and Emerging Diagnostic Technologies
The 2022 multi-country mpox outbreak spurred the development of numerous mpox and pan-orthopoxvirus diagnostics, which are now at various stages along research, development, or regulatory approval pathways. These include assets based on loop-mediated isothermal amplification (LAMP) technology, rapid diagnostics, serological assays, and a GeneXpert test; their potential effectiveness for smallpox has not been assessed. Numerous non-variola tests (e.g., mpox and pan-orthopox) developed by academic and commercial developers and used with FDA enforcement discretion policies could play an important role in a future smallpox event (FDA, 2023c). Many of these tests have been described in the literature, including real-time PCR assays used for mpox and an enzyme-linked immunosorbent assay (ELISA), for both IgG and IgM, used on subjects from Wisconsin, which was the epicenter of the 2003 U.S. mpox outbreak (Hammarlund et al., 2005; Karem et al., 2005; Y. Li et al., 2006). Any work which uses live variola virus to validate smallpox diagnostics must also be approved by the World Health Organization (WHO) Advisory Committee on Variola Virus Research (ACVVR). The lack of a sustained commercial market has likely deterred ongoing commitments for commercial assay developers.
Develop Point-of-Care and Point-of-Need Tests
Point-of-care (POC)/point-of-need (PON) tests for smallpox remain elusive.2 LAMP diagnostic assays, a relatively recent development in nucleic acid amplification, were developed and improved on during the 2022 mpox response and might be useful for POC testing in the event of a smallpox
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2 POC test: clinical lab testing conducted close to the site of patient care where treatment will be provided. PON test: that provided at a location where an ill/exposed person can go to be tested and, if positive, go to a care site if needed. The PON test may be much more widely accessible than a POC test.
outbreak, but they would need further real-world evaluation against the outbreak-causing strain (Z. Li et al., 2023). After 2022 FDA issued Emergency Use Authorizations (EUAs) for additional testing platforms, such as the Cepheid mpox assay (FDA, 2023d). Such advancements could aid in the development of future variola diagnostics particularly if developed for smaller multiplex platforms and equipment used within the clinical settings. These and other POC assays using protein or antigen-detection may be useful for initial testing to inform isolation and other public health decisions (Lewis, 2023; Meyer et al., 2020). Additionally, as has been noted with other high consequence pathogens, POC tests may also have a role to play in early evaluation of patients in resource scarce settings as well as use in mobile testing (Bhadelia, 2015; Castillo-Leon et al., 2021; Dhillon et al., 2015). Deployment of POC testing would need to be accompanied with patient and provider education about the limitations and use cases for these technologies, in partnership with public health authorities (Kost, 2021). Given the high public health concern regarding smallpox cases, rapid diagnostics aimed at the consumer are unlikely to have a role in most smallpox outbreaks.
Expand PCR Assays and Platforms
As discussed in Chapter 1, a limiting factor to expanding testing during the 2022 mpox multi-country outbreak was a lack of diverse testing assays and platforms at the beginning of the outbreak. Expansion of PCR assays and platforms, such as the development of pan-orthopoxvirus assays or variola-specific assays on a variety of platforms, could help reduce dependence on a single product, potentially reducing testing bottlenecks and increasing test accessibility. Additionally, platforms that can perform testing at different biosafety levels and at field sites would be useful in improving global access to testing.
The Non-Variola Orthopoxvirus Real-Time PCR Primer and Probe Set is a pan-orthopox DNA test approved by FDA in 2022 to qualitatively detect all orthopoxviruses, except variola and Alaskapox viruses (FDA, 2024a; Rogers et al., 2024). The test cannot identify the specific non-variola orthopoxvirus that may be causing disease. This test was used for laboratory diagnosis during the 2003 U.S. mpox outbreak response and has been used extensively since 2022. Additional PCR assays tailored for a range of platforms can increase the diversity of products available during an outbreak and improve the ability to bring testing closer to the patients, while avoiding testing and manufacturing bottlenecks.
In a potential outbreak, it is likely that the first patients evaluated for smallpox will already have rash illness and will have been infectious to others for several days prior to recognition. PCR testing can produce reliable results rapidly and is commonly used on lesion-based
samples including exudate, roofs from lesions, and lesion crusts (CDC, 2017b). During the mpox response, nucleic acid testing also found mpox in oral and naso-pharyngeal specimens, rectal tissue, semen, saliva, urine, and feces (Peiró-Mestres et al., 2022). Although the pathogenic mechanism for this observation may differ between clade IIb monkeypoxvirus infection and variola virus infection, this compares to reports of finding variola virus in saliva, conjunctiva, and urine in patients with hemorrhagic smallpox (Sarkar et al., 1973). Expanded availability of PCR testing in other sample types might allow for earlier disease identification and containment, making it the preferred method for variola testing. Identification of biomarkers for presymptomatic detection of smallpox patients and improvements to methodologies for automatic extraction methods for PCR testing could also be useful.
Antibody testing utility is low, as for all acute illnesses, due to the time lag in antibody development, which results in poor prospects for early detection. There could be utility in using antibody titers as correlates of protection in those with prior vaccination and to understand population-level exposures and immunity to smallpox (Moss, 2011).
Genomic and Environmental Surveillance
While this report is primarily concerned with clinical screening and diagnostic tools that can support smallpox readiness and response, population-level and environmental surveillance tools and systems can also support preparedness and may rely on the same kinds of technologies and technological advances critical to clinical assay development. Genomic and environmental surveillance can be used to monitor shifts in circulating poxvirus genomes; information about such shifts may support predictions of increased or decreased virulence or transmission. Genomic surveillance during an outbreak supported by bioinformatics software, such as Nextstrain, can also provide critical feedback on real-time viral evolution (Hadfield et al., 2018).
The environmental detection program, BioWatch, operated by the Department of Homeland Security, is designed to provide early alerts concerning aerosolized pathogens of concern by sampling air in strategic locations throughout the United States and coupling this with laboratory testing of the samples (IOM and NRC, 2011). Wastewater surveillance systems for disease detection were built in response to COVID-19 and were used for the detection of mpox, mostly using PCR-based tests (Oghuan et al., 2023). Future expansion of wastewater surveillance to aircraft-based networks could potentially act as an early warning system (J. Li et al., 2023). Metagenomic approaches to surveillance hold the potential to improve detection of pathogens that may not be normally suspected in a non-endemic area (Ko et al., 2022;
Sharma et al., 2023). Additionally, viral genomics and phylogenetic analyses have become important parts of public health investigations and are critical to stay ahead of pathogen evolution for emerging and priority pathogens and would be a critical component in the public health toolbox in case of a smallpox outbreak (Di Paola et al., 2020; Saravanan et al., 2022).
Conclusions on Diagnostics, Detection, and Surveillance
(2-1) Tests that can more accurately detect smallpox and other orthopoxviruses than those available today are needed; efforts should focus on (1) adapting multiplex nucleic acid assays for new platforms and field settings, (2) developing forward-deployed (POC/PON) assays to enhance equitable access to tests, including protein or antigen-based tests to rapidly test and isolate infected patients, (3) identifying FDA-approved serologic assays to assess individual and population levels of immunity against smallpox and history of related exposures, (4) validating nucleic acid testing using a variety of clinical samples, (5) developing different categories of laboratory tests for different biosafety levels, and (6) supporting a global network of laboratories to detect, diagnose, and conduct surveillance in humans and the environment.
VACCINES
As observed in the early phases of the smallpox eradication program, robust surveillance and containment strategies implemented concurrently with ring vaccination—the targeted vaccination of contacts of cases, contacts of contacts, and others at high risk of exposure—was paramount to mitigating or eliminating human-to-human transmission (IOM, 2002; Lane, 2006). A ring-vaccination-based containment approach was effective to eradicate naturally occurring smallpox due to the long incubation period of smallpox (and mpox) in humans. This made ring vaccination for smallpox and mpox, even after exposure, epidemiologically, clinically, and economically efficient at mitigating or eliminating illness and subsequent transmission compared with mass vaccination (Foege, 2011; Sah et al., 2022). Ring vaccination is also still recommended by both WHO and CDC as the first-line response strategy for a smallpox outbreak (CDC, 2019b; WHO, 2023). However, both CDC and the WHO Strategic Advisory Group of Experts on Immunization (SAGE) note that the scale of vaccination (from ring vaccination to mass vaccination) may be determined based on risk and the outbreak characteristics and the groups needing vaccinated.
For a bioterrorist attack, particularly one occurring in a populous city, mass vaccination post-event, or following an immediate response to contain spread, has also been discussed as an effective strategy. Models and simulation exercises from the early 2000s highlighted logistical challenges and the
inability for ring vaccination to be scaled rapidly enough post-event to be effective (Bicknell, 2002; Kaplan et al., 2002). In the early 2000s, preemptive mass vaccination also carried greater risk of adverse events, compared to the limited benefits afforded when the threat of a bioterrorist attack remained small (Fauci, 2002).
Uncertainties in the expected need to provide smallpox vaccine for the entire population makes it reasonable to plan for scenarios where the entire population needs vaccination. However, planning considerations would need to reflect more recent population characteristics and increasing potential for de novo synthesis or deliberate engineering to cause a smallpox event (see Chapter 2).
The Institute of Medicine’s (IOM’s) previous reports Live Variola Virus: Considerations for Continuing Research (2009) includes a detailed summary of the history of smallpox vaccine development—from early vaccine development to post-eradication era—and describes how the development of smallpox vaccines has progressed in major phases (first-, second-, third-generation) (IOM, 2009). The effectiveness of vaccines developed post-eradication to prevent disease, transmission, hospitalization, or death in the event of a smallpox or novel orthopoxvirus outbreak will remain unknown due to the inability to test vaccine effectiveness against active smallpox infections. Licensure for newer vaccines must rely on non-inferiority data for immunogenicity and protection in animal challenge models compared with first-generation or second-generation smallpox vaccines. This challenge also highlights the importance of being able to rapidly deploy adaptable clinical trials if an outbreak does occur (see Chapter 4 for more details on research readiness).
Vaccine Options and Utility
Currently, the U.S. stockpile maintains three different smallpox vaccines, all of which are based on vaccinia virus: two types of live, replicating virus vaccines; and an attenuated live, non-replicating vaccine as well as ancillary supplies for their administration (Table 2-1). The two types of replication-competent vaccinia virus vaccines in the SNS are Smallpox (Vaccinia) Vaccine, live (ACAM2000), which is a second-generation vaccine based on the first-generation vaccine Dryvax, and Aventis Pasteur Smallpox Vaccine (APSV), or WetVax, which is a first-generation vaccine (i.e., liquid formulation of calf-lymph-origin vaccinia virus vaccine) that was manufactured from 1956 to 1957 and has been maintained at −20°C (CDC, 2019a; Petersen et al., 2015). APSV is intended to be used only after ACAM2000 is exhausted, its potency is verified, and if approved for use in a smallpox emergency under an appropriate regulatory mechanism, specifically an EUA or an investigational new drug (IND) application (CDC, 2022). Live, replicating vaccinia virus vaccines are expected to be used to contain
TABLE 2-1 Summary of Smallpox Vaccines in the U.S. Strategic National Stockpile
| Product Name | Characteristics | Approved Usage | Formulation and Storage |
|---|---|---|---|
| APSV (Wetvax) | 1st generation Live, replicating vaccinia virus, NYCBOH-derived strain. |
|
Formulation contains live vaccinia virus in 50% glycerol, 0.4% phenol, 0.00017% Brilliant Green, and frozen at −20°C. |
| ACAM2000 | 2nd generation Live, replicating vaccinia virus, NYCBOH-derived strain. |
|
Lyophilized (1 × 108 pfu/ml when reconstituted), diluent is 50% Glycerin USP and 0.25% phenol USP in water, stored at −20°C. |
| MVA-BN (Imvamune, Imvanex, JYNNEOS) | 3rd generation Live, non-replicating vaccinia virus, MVA strain. |
|
Liquid, 10mM Tris, 140mM sodium chloride, host cell DNA (≤20mcg) and protein (≤500mcg), benzonase (≤0.0025mcg), gentamicin (≤0.400mcg), and ciprofloxacin (≤20.005mcg), stored frozen at −25°C to −15°C. |
SOURCES: Adams (2023); FDA (2021a, 2022); Petersen et al. (2015)
transmission following a deliberate or accidental release of smallpox, as well as for a post-event vaccination program (Petersen et al., 2015). These vaccines are also the predominant type among the current cached vaccine assets and are intended to be available in sufficient quantity for the entire U.S. population (Adams, 2023).
Non-replicating vaccines were acquired more recently to provide an alternative for individuals contraindicated to receiving replicating smallpox vaccine. Modified vaccinia Ankara vaccine-Bavarian Nordic (MVA-BN), marketed in the United States as JYNNEOS and elsewhere as Imvamune, or Imvanex, is a third-generation live, non-replicating vaccine developed in collaboration with the U.S. government. MVA-BN was developed post-eradication, and therefore not tested in a smallpox endemic setting. Studies using first-generation vaccine as a challenge, found that MVA-BN vaccination prior to vaccination with Dryvax decreased the primary cutaneous reaction and decreased the time to healing, suggesting that MVA-BN would be protective against smallpox (Frey et al., 2007; Parrino et al., 2007; Pittman et al., 2019; Seaman et al., 2010). A phase 3 trial also observed fewer adverse events among those who received two doses of MVA-BN compared to individuals who only received Dryvax (Pittman et al., 2019). The Department of Health and Human Services (HHS) has procured MVA-BN for the SNS at smaller volumes than first- and second-generation vaccines and intends to use it primarily for populations with contraindications to those vaccines (Wolfe, 2023). Additionally, the utility of MVA-BN in a ring vaccination strategy for immediate containment has not been demonstrated due to the use of a two-dose regimen. In 2023 the Advisory Committee on Immunization Practices updated recommendations for the use of MVA-BN in adults at risk for mpox, and the Strategic Advisory Group of Experts on Immunization recommended that WHO add MVA-BN to its stockpiles and review protocols for smallpox (CDC, 2023a; WHO, 2023).
The level of humoral antibody needed to provide protection against smallpox following vaccination is unknown. Data from eradication efforts suggested that a neutralizing antibody titer of more than 1:20 or more than 1:32 resulted in some level of protection against symptomatic illness (Mack et al., 1972). Currently licensed vaccines could not, for ethical reasons, include human vaccine efficacy trials. Therefore, vaccines were licensed using neutralizing antibody titer equivalence. The true efficacy as well as the duration of immunity is unknown, as was the case during the pre-eradication era. It is believed that persons who had past exposure to variola virus or received only primary immunization would have lifelong protection from fatal infection (Taub et al., 2008).
Box 2-1 provides an overview of the characteristics of first-, second-, and third-generation vaccines as well a placeholder for potential fourth-generation vaccines, which are expected to provide improvements in safety, efficacy, administration, or other key characteristics.
BOX 2-1
Smallpox Vaccine Summary and Terminology
First Generation (e.g., Dryvax, APSV)
- Live vaccinia virus vaccines developed before and during the smallpox eradication effort.
- Typically produced by infecting large animals and collecting, purifying, and lyophilizing the lymph exudate.
- Administered percutaneously using a bifurcated needle as a single dose.
- Characteristic “take,” or scar develops at vaccination site.
- Estimated high efficacy rates (typically >91%).
- Side effects and adverse events vary by virus strain.
- Multiple contraindications, especially for those who are immunocompromised or have skin or heart conditions.
Second Generation (e.g., ACAM2000)
- Live vaccinia virus vaccines developed using clonal derivatives of 1st generation vaccines (Dryvax).
- Produced in mammalian cell culture or in embryonated eggs using good manufacturing practice (GMP) methods.
- Administered percutaneously using a bifurcated needle as a single dose.
- Characteristic “take,” or scar develops at vaccination site.
- Similar immunogenicity to 1st generation vaccines.
- No efficacy data available against smallpox (developed after eradication).
- Fewer side effects and adverse events than 1st generation vaccines.
- Multiple contraindications, especially for those who are immunocompromised or have skin or heart conditions.
Third Generation (e.g., MVA-BN)
- Vaccines containing highly attenuated live vaccine or non-replicating strains of vaccinia virus.
- Produced in mammalian cell culture, embryonated eggs, or Chicken Embryo Fibroblast cells (e.g., for MVA-BN specifically) using GMP methods.
- Administered by injection.
- MVA-based formulations require two doses, at 0 and 4 weeks, do not induce formation of a “take.”
- Immunogenicity after all doses (i.e., non-inferior to 1st and 2nd generation vaccines).
- No efficacy data available against smallpox (developed after eradication).
- Far fewer side-effects and adverse events than 1st and 2nd generation vaccines.
- Relatively few contraindications. Safely administered to individuals with HIV, cancer patients, organ transplant recipients, and individuals on immunosuppressive therapies.
Fourth Generation (none currently licensed or stockpiled; in clinical development only)
- Potential subunit or nucleic acid vaccines (protein, peptide, mRNA) lacking replicating vaccinia virus.
SOURCES: CDC (2023c); Kennedy and Gregory (2023); Petersen et al. (2015).
Gaps and Pain Points
Safety Concerns and Adverse Events
The safety profiles of ACAM2000 and APSV (Wetvax) are expected to be similar to Dryvax (first-generation vaccine), as both vaccines are based on the New York City Board of Health (NYCBOH) strain used in Dryvax (Petersen et al., 2015). Adverse events, though rare, can be severe and life-threatening (Table 2-2). Historical data indicate that first-generation vaccines were associated with at least 1 death per million primary vaccinations given, with the higher frequency of adverse events arising following primary vaccination and in younger children (Belongia and Naleway, 2003). It is important to note however that these data come from a period when smallpox vaccines were administered within the first months of life, knowledge of cell-mediated immunity was lacking, and some contraindications could not be accurately diagnosed in young infants. More recently, a voluntary smallpox vaccination program implemented in 2002 using second-generation vaccines among military personnel and civilians was discontinued by June 2003 due to cases of myo/pericarditis, concerns of viral shedding and risk of vaccinia infection to others, and at least three deaths (IOM, 2005).
CDC’s clinical use guidelines for smallpox vaccine indicate that there are no absolute contraindications for a smallpox vaccination in a post-event setting but that there are certain “relative” contraindications due to the possibility of adverse events associated with certain conditions (Table 2-2). CDC developed its recommendations for smallpox use by weighing risk for smallpox infection, risk for an adverse event following vaccination, and benefit from vaccination (Petersen et al., 2015). Relative contraindications include atopic dermatitis (eczema), HIV infection (CD4 cell counts of 50–199 cells/mm3), other immunocompromised states, and allergies to the vaccine or vaccine components. The adverse events can range from generalized vaccinia, a rash resulting from spread of the replicating virus from the injection site, to myopericarditis and death. The rates of these events are variable and, in some cases, unknown (Table 2-2).
TABLE 2-2 Potential Adverse Events Associated with Smallpox Vaccination with Replicating Vaccine
| Adverse Event | Description | Historical Risk of Complication |
|---|---|---|
| Auto-inoculation | Transfer of the virus from the injection site to another site on the body or to another individual. Lesions occur on new site with the same progression as the vaccine site. If transferred to the eye, vaccinia keratitis, corneal scarring, and blindness can occur. | 0–1647 per million primary vaccinations 0–675.7 per million revaccinations |
| Bacterial infections | Bacterial infection of the vaccination site; typically, staphylococci or group A streptococci. Can be minimized by appropriate vaccine site care. Typically responds well to antimicrobials. | No historical frequency data reported. |
| Congenital vaccinia | Infection in utero can occur after vaccination of pregnant women. Exceptionally rare and results in stillbirth or death of the newborn shortly after birth. | ~10–100 per million vaccinations |
| Death | Typically, due to post-vaccinal encephalitis (fatality rate = 25%), progressive vaccinia (fatality rate nearly 100% and eczema vaccinatum (10% before vaccinia immune globulin [VIG], 2% afterward). | Historically reported as 1–2 deaths per million primary vaccinations. Often in young children or those with unrecognized defects in T cell immunity. |
| Eczema vaccinatum | Large areas of the skin become covered in confluent lesions. Untreated, this can lead to systemic symptoms and septic shock. VIG treatment is effective if given within 1–2 days of symptom onset. This adverse event is seen in recipients with underlying skin conditions (eczema, atopic dermatitis) even without active disease. | 8.1–96.8 per million primary vaccinations 0–13.5 per million revaccinations |
| Generalized vaccinia | Viremia and rash spreading from vaccination site or occurring elsewhere on the body. Most cases resolve without treatment. VIG can be used, as can treatment appropriate for known immune abnormalities. | 20.8–387.1 per million primary vaccinations 0–9.0 per million revaccinations |
| Myopericarditis | Inflammation of the heart muscle or the pericardium/pericardial space. Can be accompanied by shortness of breath, chest discomfort, or pain. Symptoms occur 4–30 days after vaccination. | Not assumed to be vaccine-related and therefore not well documented during eradication era. |
| Adverse Event | Description | Historical Risk of Complication |
|---|---|---|
| Post-eradication data from military populations indicate a rate of 80–160 cases per million primary vaccine recipients. Reports focusing on civilian populations have a higher rate ~500 cases per million vaccinees. |
||
| Non-infectious rashes (erythema multiforme) | Generalized rash occurring 1–2 weeks after vaccination. Most resolve spontaneously and can be managed by over-the-counter treatments. Rarely, they can result in hospitalization. Stevens-Johnson syndrome and death have also been reported. | 51.5–164.6 per million primary vaccinations 2.1–10 per million revaccinations |
| Post-vaccinal encephalitis | Encephalitis occurs 1–2 weeks after vaccination, causing one or more of the following: headache, drowsiness, vomiting, muscular weakness, ataxia, paralysis, coma, convulsion. Has a fatality rate of 25%. In 50% of survivors, some degree of neurologic damage is present, which can be permanent. No known predictors of susceptibility markers. VIG is not effective. | 1.5–176.5 per million primary vaccinees 0–2 per million re-vaccinees |
| Progressive vaccinia (disseminated vaccinia, vaccinia necrosum) | Characterized by a failure of vaccine site to heal, progressive spreading of the lesion in the absence of inflammation, viremia leading to additional lesions on distal sites that follow the same progressive spread. Believed to be a result of a defective or inadequate T cell response. Bacterial infection is common, and patients can suffer from toxic or septicemic shock. Treatment with VIG was used in the 1960s with partial success. Recently developed antiviral drugs (e.g., Cidofovir or ST-246) may be effective. | 0–6.9 per million primary vaccinations 0–6 per million revaccinations |
SOURCES: Aragón et al. (2003); Arness et al. (2004); Engler et al. (2015); Lane et al. (1969, 1970); Morgan et al. (2008); Murphy et al. (2003); Neff et al. (1967a,b); Ratner et al. (1970); Ryan et al. (2008); Tack et al. (2013).
Due to this potential for severe adverse events in certain immunocompromised persons, CDC recommends that people with relative contraindications be vaccinated with MVA-BN. Clinical trials involving healthy and high-risk populations, such as those with HIV infection, have reported no cases of myo-/pericarditis after vaccination with MVA-based vaccines, whereas myo-/pericarditis was observed with the use of first-and second-generation live smallpox vaccines (Overton et al., 2015; Zitzmann-Roth et al., 2015). The rates and severity of mostly mild, self-limiting common side effects (e.g., headache, fever, myalgia, regional lymphadenopathy) are broadly similar for MVA-BN and ACAM2000.
The main disadvantages of MVA-BN in a post-event setting are that it requires two injections given 4 weeks apart, and, unlike replicating vaccines, its real-world efficacy against smallpox is unknown and has been assessed only through animal models, serological endpoints, and non-inferiority studies. Vaccination campaigns using non-replicating MVA-BN to vaccinate persons at risk of mpox provide additional valuable data and demonstrate effectiveness rates for two doses ranging from 66 percent to 86 percent (Dalton et al., 2023; Deputy et al., 2023; Rosenberg et al., 2023). While MVA-BN is safe for immunocompromised persons and should provide protection for those at risk of eczema vaccinatum, effectiveness in severely immunodeficient populations is expected to be lower due to inability to mount an effective immune response after vaccination (CDC, 2023b). However, it is unknown whether an exposure dose–response or exposure based on differing modes of transmission (e.g., sexual contact related disease vs. other modes of mpox transmission) is associated with observed vaccine efficacy.
Reliance on the U.S. Strategic National Stockpile and Concerns with Manufacturing Capacity
Dependence on the SNS smallpox vaccine stockpile as the primary form of smallpox readiness has required that sufficient inventory be maintained to respond to a smallpox event where the entire U.S. population could potentially require vaccination. This explains, in part, the large proportion of spending that has gone to smallpox MCM. Under the current stockpiling strategy, ACAM2000 will be the primary vaccine used in a smallpox response scenario. While older second-generation vaccines in the SNS are thought to be quite stable, their potency is less well understood, and MVA-BN had been intended to be used for only a subset of the population. While quantities of stockpiled vaccine doses are not released publicly, research estimates put the U.S. immunocompromised population (the main population for which MVA-BN is intended) at approximately 15 percent of the population, more than 49 million people, rising to about 38 percent if their household contacts are included (Adams, 2023; Carlin et al., 2017).
Notably, this figure does not include the population with post-acute sequelae of SARS-CoV-2 (long COVID), who appear to have immune dysfunction following COVID-19 infection, though the committee found no evidence that such a state would be an absolute or relative contraindication to live smallpox vaccination (Phetsouphanh et al., 2022). The risks and benefits of live vaccine for the heterogeneous immunocompromised population must be weighed. Finally, a likely approach to contain a smallpox outbreak would be to use both vaccines and therapeutics. However, the potential interactions between vaccines and antivirals if used concomitantly are not well understood (Russo et al., 2020).
The choice to focus on stockpiling vaccines has been driven, in part, by limited production capacity, which makes it difficult to rapidly manufacture additional smallpox vaccine doses. Smallpox vaccines are not mass-market vaccines that are produced in a routine manner using widely available ingredients and common manufacturing processes. Ramping up the production of smallpox vaccines to meet a sudden demand would require procuring potentially limited raw materials and navigating multiple regulatory and safety hurdles.
Future Opportunities and Emerging Vaccine Technologies
Opportunities for newer vaccines include the development of those with fewer contraindications, broad protection, single-dose administration, and stable shelf-life. As noted by the WHO ACVVR, the development of “scalable less-reactogenic vaccines” with improved efficacy and durability of protection would be “essential for the control of an outbreak of smallpox in the current context, should it recur” (WHO, 2024). Research on fourth generation smallpox vaccines could explore potential subunit or nucleic acid vaccines (protein, peptide, mRNA) lacking replicating vaccinia virus.
Explore Innovative Vaccine Platforms
Safe and effective COVID-19 vaccines were made in under a year since the start of the pandemic due to decades of research on mRNA vaccines and U.S. government investment in mRNA platform technology (NIH, 2022). Smallpox vaccine development could benefit from investments in novel, rapid vaccine platforms such as mRNA, viral vectors, and protein nanoparticles. Like for most potential vaccine targets, mRNA technology may be a useful approach; however, the utility of this approach, specifically for orthopoxviruses remains uncertain. There is work underway evaluating novel mRNA constructs, however (Hou et al., 2023), and the National Institutes of Health (NIH) is supporting the development of mRNA-based vaccines for
orthopoxviruses (Freyn et al., 2023; HHS, 2023). Modified vaccinia Ankara (MVA) has also shown utility as a vaccine vector for non-orthopoxvirus pathogens (e.g., HIV, malaria, Ebola, tuberculosis, MERS-CoV) as well as in oncolytic virotherapies (Lin et al., 2023; Orlova et al., 2022). While any vaccine using the MVA backbone would provide some protection against smallpox and other orthopoxviruses, the utility of recombinant MVA vaccines to contain a smallpox emergency is uncertain.
Develop Immunobridging Strategy
Development of an immunobridging strategy would be important to allow for useful cross-vaccine comparisons, especially as there is no way to effectively test a smallpox vaccine for efficacy against smallpox. This type of approach has been considered for COVID-19, filovirus as well as for seasonal influenza vaccines (Gruber et al., 2023; Khoury et al., 2023). Basic clinical trials for safety and immunogenicity can be accomplished through routine clinical development strategies along with relevant antecedent preclinical studies. Further work into the utility of fourth-generation vaccines that use multi-vaccine platforms and immunobridging strategies would allow for greater diversity in the vaccine products that are available. Diversification of MCM products in the SNS is highlighted in Chapter 4.
Conclusions on Vaccines
(2-2) Smallpox vaccines that have improved safety across different population subgroups and are available as a single dose would support faster and more effective response to contain smallpox and other orthopoxvirus outbreaks. The development of novel smallpox vaccines using multi-vaccine platforms (i.e., use common vaccine vectors, manufacturing ingredients, and processes) would improve the capacity for rapid vaccine production in response to a smallpox event and reduce the need for stockpiling in the SNS at current levels.
THERAPEUTICS
Initiated in the late 1990s, the initial approach to developing smallpox treatments was to screen small molecule compounds and further develop those that demonstrate activity across orthopoxviruses (OPXVs) (Baker et al., 2003). More recent efforts have evaluated anti-variola and anti-orthopoxvirus activity with a focus on biologics, such as monoclonal antibodies, antibody cocktails, and vaccinia immunoglobulin (VIG), as well as antivirals with distinct mechanisms of action (Martins, 2023). Table 2-3 presents a summary of smallpox antivirals with additional details on their mechanisms of action, indications, bioavailability, and storage.
TABLE 2-3 Summary of Smallpox Therapeutics in the U.S. Strategic National Stockpile
| Product Name | Mechanism of Action | Indication | Bioavailability | Formulation and Storage |
|---|---|---|---|---|
| Tecovirimat (ST-246/TPOXX) | Orthopoxvirus-specific inhibition of viral spread from cell to cell by targeting p37, a major envelope protein required for envelopment and excretion of extracellular forms of the virus. |
|
|
Capsules stored in the original bottle at 20°C to 25°C (68°F to 77°F); excursions permitted 15°C to 30°C (59°F to 86°F). Injection stored at 2°C to 8°C (36°F to 46°F). FDA had initially approved a shelf-life extension of tecovirimat injection from 24 months to 42 months for some lots (FDA, 2023b). |
| Brincidofovir (CMX001/Tembexa) | Pro-drug of cidofovirf; following phosphorylation of the prodrug to the active form cidofovir diphosphate which targets the orthopoxvirus DNA polymerase, causing disruption of replication of the virus. |
|
Oral suspension—expiry 30 months from the date of manufacture when stored at 20°C to 25°C (68°F to 77°F) (FDA, 2021b). Tablets—expiry 48 months from the date of manufacture when stored at 20°C to 25°C (68°F to 77°F). |
| Vaccinia Immune Globulin Intravenous Human (VIGIV) (CNJ-016) | Passive immunity for individuals with complications to vaccinia virus following vaccination; exact mechanism of action is not known. |
|
|
Stored frozen at or below 5°F (≤ –15°C) or refrigerated at 36°F to 46°F (2°C to 8°C); if received frozen, use within 60 days of thawing at 36°F to 46°F (2°C to 8°C). |
NOTES: Cidofovir (Vistide), another antiviral not currently stockpiled, targets orthopoxvirus DNA polymerase, causing disruption of replication of the virus. The licensed indication is for cytomegalovirus (CMV) retinitis and does not have licensed indication for OPXV/VARV therapy. However, it can be used “off label.” Cidofovir was used in therapy of mpox in those with compromised immune systems/uncontrolled HIV during the 2022–2023 multi-country outbreak, as this treatment was commercially available. Potential side effects include renal toxicity.
CDC = U.S. Centers for Disease Control and Prevention; EA = expanded access; EIND = emergency investigational new drug application; IND = investigational new drug application; OPXV = orthopoxvirus; VARV = variola virus.
a NIH/NIAID-sponsored: Phase 3 randomized, placebo-controlled, double-blind study to establish the efficacy of tecovirimat for the treatment of people with laboratory-confirmed or presumptive human monkeypox virus disease (HMPXV) [NCT05534984].
b Open label for pregnant or breastfeeding persons; those with severe immune suppression, significant skin conditions, or severe disease.
c For both mpox and smallpox for Department of Defense–affiliated personnel [NCT02080767].
d There are no registered clinical trials for brincidofovir at this time, could be used under EIND through CDC.
e Phase 3 trial completed 12/2022 with posted results [NCT01143181].
f Cidofovir was used in therapy of mpox in those with compromised immune systems/uncontrolled HIV during the 2022-2023 multi-country outbreak, as this treatment was commercially available.
SOURCES: Chan-Tack et al. (2021); Emergent BioSolutions Canada Inc. (2018); Gilead Sciences (2000); Grosenbach et al. (2011, 2018); Huston et al. (2023); Jordan et al. (2010); Merchlinsky et al. (2019); Yang et al. (2005).
Potential smallpox therapeutics can only be tested in vitro against variola virus or by using nonhuman animal models or surrogate viruses (e.g., non-variola virus). Such testing data, coupled with pharmacokinetic and safety data in humans, has allowed for FDA approval of tecovirimat and brincidofovir under the Animal Rule pathway (FDA, 2023a). Additionally, in many studies assessing the efficacy of antivirals against mpox in small animals, therapeutics were administered prior to the onset of rash illness (Hutson and Damon, 2010). More recent efforts have looked at therapeutic efficacy in nonhuman primates when initiated after rash onset following exposure to mpox (Russo et al., 2018). Because clinical guidelines rely on the presentation of rash illness to collect specimens for testing and to identify probable smallpox cases, the effectiveness of these therapeutics against the most likely early case presentations (i.e., people with a smallpox rash) is uncertain. Experience with the mpox outbreak revealed that those with severe immunosuppression required extended courses of treatment, often using multiple therapeutics (Pinnetti et al., 2023).
Therapeutics Options and Utility
Smallpox antiviral research and development efforts to date have resulted in two antiviral agents that are FDA approved for the treatment of human smallpox in adult and pediatric patients: tecovirimat and brincidofovir (CDC, 2023d,e). These products have different mechanisms of action and oral bioavailability (see Table 2-3). Both tecovirimat and brincidofovir were approved under the Animal Rule and must be requested from the SNS, as neither is commercially available. Tecovirimat is efficacious as a post-exposure therapeutic treatment of orthopoxvirus, specifically mpox, and its utility as a pre-exposure prophylaxis in high-risk humans is likely following observed therapeutic benefits in nonhuman primates evaluated in a prelesional and postlesional setting (Mucker et al., 2013). Cidofovir, a commercially available drug approved by FDA in 1996 for the treatment of AIDS-related cytomegalovirus (CMV) retinitis, was also used “off label” to treat mpox in immunocompromised individuals during the 2022 multi-country mpox outbreak (Siegrist and Sassine, 2023). Cidofovir has a more severe side effect profile than brincidofovir and is only recommended when the latter is not readily available.
Gaps and Pain Points
Lack of Diverse Antivirals
The two approved antivirals have independent mechanisms of action and are not antagonistic of each other. While brincidofovir slows DNA
synthesis, tecovirimat inhibits membrane protein p37 that is essential for formation of an enveloped virus, so both could be used in combination (Siegrist, 2023). However, the effectiveness and utility of combination therapies to improve treatment efficacy and reduce the potential of anti-viral resistance are yet to be determined (P. Li et al., 2023). Periodic evaluation of compound libraries may ultimately yield additional promising therapeutics. Additionally, it would be ideal to have antivirals that act upon different stages of the virus lifecycle, (e.g., targeting specific virion components or other steps of viral infection) to mitigate the potential for antiviral resistance (Delaune and Iseni, 2020). Finding drugs that target and kill only the virus without damaging host cells is also a challenge because viruses use the host’s cell to replicate (Kausar et al., 2021). In addition to considering the mechanisms of action, diverse modes of antiviral administration might provide increased protection. In one study, aerosolized cidofovir was found to protect mice against an otherwise lethal challenge of aerosolized cowpox virus (Bray and Roy, 2004). A similar approach could be possible with aerosolized brincidofovir as well.
Concerns with Antiviral Resistance and Adverse Events
Antiviral resistance is a significant concern for the current antivirals. Stepwise resistance to cidofovir is known to occur when treating CMV, with moderate resistance with single mutations and a higher level of resistance with multiple mutations (Siegrist and Sassine, 2023). Notably, cidofovir-resistant virus is less virulent than wild type (CDV-sensitive) virus. Whether this would occur in the setting of monkeypox (MPXV) or VARV is unknown. With tecovirimat and brincidofovir, single amino acid resistance mutations have been observed, with questionable impacts on viral fitness (Foster et al., 2017; Smith et al., 2023).
Adverse events, such as serious renal toxicity, have been observed during treatment of CMV retinitis infections with cidofovir (Skiest et al., 1999). Additionally, intravenous tecovirimat is not recommended for patients with severe renal impairment (CDC, 2023d).
Future Opportunities and Emerging Therapeutic Technologies
Research and development efforts in the past 10 years have often focused on re-purposing non-vaccine biologics and antibody-derived therapeutics:
- Vaccinia immune globulin intravenous (VIGIV) is a polyclonal antibody therapy and FDA-licensed biological agent (since 2005) for the treatment or modification of complications resulting from vaccinia smallpox vaccination (CDC, 2023c). There is potential to repurpose
- BFI-753 (Biofactura), a two-antibody cocktail to treat smallpox, is in late preclinical development, with an anticipated IND application submission in 2025 (Martins, 2023).
- Preclinical studies on potential therapeutics targeting cellular proteins or systems required for the virus lifecycle, such as imatinib mesylate (Gleevec®), have not progressed past early-stage research and development efforts, as concerns remain about imatinib’s potential for treating smallpox (Ananthula et al., 2018; IOM, 2009; Reeves et al., 2005, 2011). For example, animal model benefit required continuous pump infusion, indicating challenges with administration.
- ST-357 (Siga Technologies), a smallpox antiviral with a mechanism of action distinct from tecovirimat and brincidofovir that targets conserved domains in orthopoxviruses, and which is undergoing testing in animal models (Hruby, 2023).
- NIOCH-14 is a derivative of tricyclodicarboxylic acid and a precursor of tecovirimat developed and tested in the Russian Federation (ClinicalTrials.gov, 2023; Delaune and Iseni, 2020). In animal models (i.e., an Institute of Cancer Research mouse model and marmoset model using MPXV challenge), NIOCH-14 demonstrated equal efficacy to tecovirimat and purportedly is easier to produce (Delaune and Iseni, 2020; Mazurkov et al., 2016).
- Most recently, a potential for custom designed antivirals for orthopoxviruses has been demonstrated using CRISPR technologies in in vitro studies (Siegrist et al., 2020)
this biologic for treatment of orthopoxviruses in an outbreak, however VIGIV has not been considered an effective standalone therapeutic for smallpox (Jahrling 2011). In response to the 2022 multi-country outbreak, expanded access for VIGIV was provided through an Investigational New Drug (IND) Application to treat mpox infection in adults and children in concert with small molecule compounds; however, data on efficacy are limited (Gilchuk et al., 2016).
Conclusion on Therapeutics
(2-3) To treat smallpox, the following would be advantageous to develop in order to supplement the therapeutic options currently approved and stockpiled in the SNS (1) new, safer antivirals with different and diverse targets, mechanisms of action, and routes of administration that minimize damage to host cells and have a high barrier to the development of resistance; (2) combination antiviral treatments and treatments based on novel technologies and platforms (e.g., genome editing, non-conventional targets, etc.); (3) Vaccinia immune globulin intravenous (VIGIV) repurposed as part of combination therapy; (4) diverse options for non-vaccine biologics including monoclonal antibodies and antibody cocktails.
(2-4) Most mpox therapeutics were developed because of investments in smallpox therapeutics, resulting in products found to have activity against mpox. Direct investment in developing therapeutics targeting circulating orthopoxviruses could similarly benefit smallpox therapeutic preparedness and could likely have more immediate utility and potentially achieve commercial viability.
OVERARCHING CONCLUSION
Based on the evidence and findings on the current state of smallpox MCMs, the committee drew the following overarching conclusion:
The COVID-19 pandemic revealed weaknesses in the ability for the nation’s public health and health care systems to rapidly and flexibly adapt the emergency response to an unfamiliar pathogen; whereas the 2022 mpox outbreak tested existing MCMs developed primarily for smallpox to contain a less lethal orthopoxvirus. The lessons learned from both emergencies call for strengthening the nation’s laboratory response systems and further development of point-of-care diagnostics and genomics surveillance capabilities. Additionally, safer, single-dose vaccines and a diverse set of therapeutic options against smallpox would improve the U.S. readiness and response posture for immediate containment and long-term protection in a smallpox emergency.
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