Preventing Transmission of Pandemic Influenza and Other Viral Respiratory Diseases Personal Protective Equipment for Healthcare Personnel: Update 2010 (2011) / Chapter Skim
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2 Understanding the Risk to Healthcare Personnel
2 Understanding the Risk to Healthcare Personnel
Pages 29-70
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research on viral respiratory disease transmission. Studies on personal protective equipment (PPE)
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These are the equivalent of "droplet nuclei." • Inspirable particles -- particles with 10 μm ≤ da ≤ 100 μm, which can be inhaled but cannot penetrate to the alveolar region; nearly all de posit in the head airways region. Transmission Routes: • Contact transmission: o Direct contact transmission occurs when the virus is transferred by contact from an infected person to another person without a contaminated intermediate object.
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A common set of definitions accepted by the industrial hygiene, infectious disease, and healthcare communities would be most helpful in discussing future research and policy. Much of the discussion regarding influenza transmission has focused on the continuum between droplet spray and aerosol transmission, as well as on the role of contact transmission and the potential for transmission through inoculation of the conjunctivae.
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These studies show that animals develop influenza infection, and most demonstrate the possible role of aerosol transmission. Experiments performed in the 1930s demonstrated that influenza virus–naïve, asymptomatic ferrets that were caged with influenza virus–infected ferrets would subsequently develop disease and that, even in the absence of experimental infection, ferrets occasionally displayed an influenza-like illness, after which they became immune to subsequent virus inoculation (Francis and Magill, 1935; Smith et al., 1933)
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Studies examining how air flow may help prevent transmission of viral respiratory diseases in closed and crowded settings, such as an airplane, are warranted. Wong and colleagues (2010)
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found that assisting during intubation, suctioning before intubation, and manipulating the oxygen mask were high-risk activities for acquiring SARS; wearing a face mask or N95 respirator was protective. As stated throughout the 2008 report, establishing how influenza is transmitted and understanding the contribution of each mode of transmission is critical to preventing its spread and reducing morbidity and mortality due to influenza infection, especially in healthcare settings.
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Searches of bibliographic databases for studies on PPE use and transmission were broader and incorporated other viral respiratory diseases; only a few recent studies on other viral respiratory diseases were identified, however, and those are discussed and referenced in this report. Animal Studies Animal models complement epidemiological approaches by allowing the examination of influenza virus transmission from an infected to a susceptible host under well-controlled conditions.
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. In addition, evidence for transmission of influenza viruses by the aerosol route has been obtained in the ferret and guinea pig models; early work in ferrets (Andrewes and Glover, 1941)
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. When these results are compared to the high efficiency of transmission of the same virus by the aerosol route, they suggest that -- at least in the guinea pig model -- spread via fomites makes a minor contribution to the overall transmission of influenza viruses.
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Relative Transmissibility of Influenza Viruses Derived from Different Host Species Viral strain and subtype specific differences in influenza virus transmission have been observed in recent studies of animal models. One strength of both the ferret and guinea pig models is that influenza viruses adapted to human hosts generally transmit more efficiently than avian-, swine-, or mouse-adapted strains.
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. Although field studies are required to translate these findings to the human situation, they suggest that the modification of relative humidity in healthcare settings may be a means of controlling the spread of influenza virus infection.
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However, the viability of the influenza viruses was not ascertained, and therefore it is not possible to quantify the importance of the identified aerosol particles to transmission in the hospital setting. A recent study by Lindsley and colleagues (2010b)
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. Of the inactivation rates reviewed across a range of respiratory viruses (rhinovirus, RSV, coronaviruses, parainfluenza virus, avian influenza, and influenza A and B viruses)
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examined the extent of 2009 H1N1 contamination on the hands of healthcare personnel and patients and on environmental surfaces in a hospital in Mexico. The computer mouse, hands, and bed rails were all found to be positive for the influenza virus, but viability of the virus was not assessed.
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Transmission Models Although empirical studies have shown that influenza transmission is feasible by aerosol, droplet spray, and contact routes, the results of these studies have not provided a comprehensive understanding of the relative contribution of each mode in causing outbreaks. Mathematical models can be used as a method for testing hypotheses about the spread of respiratory infections (Brauer, 2009)
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The infectivity ratio assumptions played a major role in determining which method of transmission was most likely to cause disease. Assuming a 3,200:1 infectivity ratio for influenza virus deposited in the lower respiratory tract compared with the upper respiratory tract, droplet spray accounted for 58 percent of the infection risk at low salivary viral concentrations, compared to 27 percent of the risk for fomite or hand contact and 14 percent of risk from respirable particles.
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. An exposure time of less than 30 minutes was found to reduce the likelihood of transmission, as were surgical masks to reduce droplet spray transmission (assuming a reduction in the risk of contaminated air inhaled by 40 percent)
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modeled the risk of transmissible respiratory diseases in a healthcare setting. The authors provided an integrated method of examining transmission between infected individuals, contaminated environments, and direct patient-tohealthcare worker exposure, which could be used as a template for an influenza-specific model in the healthcare setting.
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Human Challenge, Observational, and Clinical Studies Infectivity, Viral Shedding, and Symptoms The timing of infection and quantity of viral shedding obviously play a role in the spread of influenza. A meta-analysis of 56 volunteer challenge studies attempted to quantify the time of peak viral shedding among healthy human volunteers (Carrat et al., 2008b)
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used a communitybased study of households to show that the bulk of viral shedding happened during the first 2 to 3 days after illness onset and only 1 to 8 percent of infectiousness occurs prior to symptom onset. Moreover, only 14 percent of cases that had RT-PCR–detectable influenza virus RNA were asymptomatic, and the quantity of viral particles was low among these cases, suggesting that asymptomatic cases are unlikely to play a large role in transmission.
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, transmission routes of SARS were examined. Major transmission modes that have been demonstrated include close contact via droplet spray or contaminated fomites with respiratory excretion.
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PPE Use to Prevent Respiratory Disease Transmission Models on the Use of PPE The effectiveness of surgical masks and N95 respirators in reducing the spread of influenza were directly modeled as a method of preventing the 2009 H1N1 (Tracht et al., 2010)
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dropped the total number of cases drastically to approximately 0.1 percent. The authors concluded that surgical masks were unlikely to impact the epidemic because of low effectiveness at reducing the spread of influenza among susceptible and infected individuals, while N95 respirators could reduce the impact of the epidemic, though they could not reduce the R0 below 1.
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Several recent randomized community intervention studies have attempted to compare face-mask use and hand hygiene or a combination of both interventions during seasonal influenza seasons. In one study, researchers randomized university residence halls housing 1,297 student participants to use of face masks, face masks with hand hygiene, or a control group for a 6-week period during the influenza season (Aiello et al., 2010)
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Last, these types of studies are unable to provide insights on the modes of transmission of influenza because face masks may block both droplet spray and direct contact inoculation from hands contaminated with influenza virus. Clinical Studies of PPE Use by Healthcare Personnel Although the benefits of vaccination are clear (Fiore et al., 2009; Treanor et al., 1999)
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A significant difference between the cases and controls was that cases were less likely to be vaccinated and were also more likely to have been exposed to a sick colleague without using PPE. In a survey of healthcare personnel participating in a medical mission and treating patients in crowded conditions, fewer cases of acute respiratory illness were noted for personnel using hand sanitizer; however, use of face masks was not reported to make a difference (Al-Asmary et al., 2007)
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The study stopped collecting data in late April 2009, with the reporting of novel H1N1 influenza A and the recommendation by the Ontario Ministry of Health and Long-Term Care that N95 respirators be used for caring for patients with febrile respiratory disease. Laboratoryconfirmed influenza was documented in 50 of the 225 nurses allocated to wear surgical masks (23.6 percent)
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Use of Face Masks and Respirators as Source Control Face masks and respirators have also been used as source control, that is, placing a mask on patients with respiratory illnesses in clinics or emergency departments to reduce the potential for disease transmission to other patients, family members, or healthcare personnel. Johnson and colleagues (2009)
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Much remains to be learned about the effectiveness of control measures to prevent transmission. Observational studies and controlled studies relevant to PPE use and transmission of influenza or other viral respiratory diseases are limited because study protocols were largely not in place for 2009 H1N1 or for recent seasonal flu periods, and studies have not provided adequate power to answer questions regarding the effectiveness of using PPE in reducing or preventing disease transmission.
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Transmission routes need to be better understood to select appropriate personal protective equipment (PPE) and better protect healthcare personnel and patients.
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In addition, the impact of environmental factors, such as UV and humidity, on influenza transmission and infection should be examined in the community and healthcare setting. Modeling studies Statistical and mathematical models need to be evaluated for their utility in prediction and inferences regarding the relative contributions of different transmission modes in varying environmental/community contexts.
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Recommendation: Develop and Implement a Comprehensive Research Strategy to Understand Viral Respiratory Disease Transmission The National Institutes of Health, in collaboration with other research agencies and organizations, should develop and fund a comprehensive research strategy to improve the un derstanding of viral respiratory disease transmission, includ ing, but not limited to, examining the characteristics of influenza transmission, animal models, human challenge stud ies, and intervention trials. This strategy should include • an expedited mechanism for funding these types of studies and • clinical research centers of excellence for studying in fluenza and other respiratory virus transmission.
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1982. Transmissibility of influenza viruses in hamsters.
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2010. Coronavirus survival on healthcare personal protective equipment.
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2009. Identification of amino acids in HA and PB2 critical for the transmission of H5N1 avian influenza viruses in a mammalian host.
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2010. Protecting healthcare workers from pandemic influenza: N95 or surgical masks?
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2010. Viral shedding and clinical illness in naturally acquired influenza virus infections.
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2006. The guinea pig as a transmission model for human influenza viruses.
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2006. Lack of transmission of H5N1 avian-human reassortant influenza viruses in a ferret model.
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1976. Survival of airborne influenza virus: Effects of propagating host, relative humidity, and composition of spray fluids.
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Presentation at the Institute of Medicine workshop on Current Research Issues -- Personal Protective Equipment for Healthcare Workers to Prevent Transmission of Pandemic Influenza and Other Viral Respiratory Infections, June 3, Washington, DC. http://iom.edu/~/media/Files/Activity% 20Files/PublicHealth/PPECurrentResearch/2010-JUN-3/Van%20Tam%20% 20Panel%201%20approved%20for%20web.pdf (accessed December 8, 2010)
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