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Contrary to Prevailing Beliefs, Aerosol may be Significant Mode of Flu Transmission

By David Press, M.D. and Eric Toner, M.D., September 28, 2006

In a review article released ahead of print in the November issue of Emerging Infectious Diseases [1], Raymond Tellier examines the evidence supporting the widespread assumption that influenza is transmitted from person to person primarily by large respiratory droplets rather than small aerosols. This distinction is important because the mode of transmission determines infection control procedures and choice of personal protective equipment (PPE) for healthcare workers.

Tellier points out that there are three established modes of transmission of the influenza virus: aerosols, large droplets, and direct contact with secretions. Aerosols are generally considered to be less than 5 μm in size, while large droplets are at least 10 μm. Both coughing and sneezing can elicit particles small enough to be aerosolized, and, due to their small size, aerosols can remain suspended in the air for more than one hour and are able to travel long distances. 

Tellier notes that in many articles and in most plans, the predominant mode of transmission is assumed to be large droplets, so little attention has been given to aerosol transmission. He argues, however, that a review of the literature contradicts this view, and asserts that there is strong experimental evidence of aerosol transmission of influenza. Epidemiological observations from several outbreaks also suggest predominantly aerosol transmission.

The prevailing logic is that use of droplet precautions in institutions has proven effective in interrupting outbreaks of seasonal influenza; therefore, droplets must be the primary mode of transmission. Tellier points out that there are several confounding or mitigating factors that render this conclusion erroneous: 1) the lack of definitive laboratory diagnosis of influenza, which may result in incorrect diagnoses of flu; 2) the lack of serological studies that would uncover otherwise missed asymptomatic cases; 3) the preponderance of studies are of seasonal influenza, which do not account for widespread partial immunity; and 4) the partial protection afforded by surgical masks. Tellier argues that because these factors have confounded the interpretation of data regarding the mode of transmission of influenza, many plans for responding to an influenza pandemic are misguided.

Several national pandemic influenza plans, including that of the U.S. Department of Health and Human Services (HHS), recommend the use of surgical masks rather than N95 respirators as standard personal protective equipment (PPE) for all healthcare workers. When used as PPE, surgical masks may protect individuals from large droplets, but are, at best, only partially effective against aerosolized particles. Aerosols require the use of more expensive and more difficult to use devices such as N95 respirators. Tellier states that unless the guidelines are revised, healthcare workers will face significant risk of high morbidity during a pandemic.

Ultimately, Tellier concludes that for purposes of pandemic planning it is not necessary to determine which mode of transmission predominates; rather, it is sufficient to show that aerosol transmission occurs with appreciable frequency, and to plan accordingly. Because he believes the evidence for significant aerosol transmission to be compelling, Tellier insists that the current guidelines must be changed to adopt N95 respirators for standard PPE for healthcare workers during a flu pandemic. The current standard, surgical masks, is not adequate.

We agree with Tellier’s conclusion. Healthcare workers must be protected during a pandemic, which makes it essential to provide them with the highest degree of protection possible. That said, it will be expensive and difficult to provide enough N95s for all of the front line health care workers who will need them for the duration of a pandemic.  Given that there is a finite supply of N95s and manufacturing capacity is limited, it is likely that they will be in short supply once a pandemic starts. Therefore, healthcare institutions should be building their stockpiles now.  

Reference

Tellier R. Review of aerosol transmission of influenza A virus. Emerg Infect Dis [serial on the Internet]. 2006 Nov .  Available from http://www.cdc.gov/ncidod/EID/vol12no11/06-0426.htm   Accessed September 27, 2006.