How is the flu is transmitted from person to person?© Evah Smit
JRC scientists challenge assumptions on spread of the flu
It may come as a surprise to many, but the precise manner in which the flu is transmitted from person to person remains unknown. The prevailing view is that large droplets expelled during coughing or sneezing are chiefly responsible for transmitting the disease. But coughing and sneezing also produces fine droplets that can remain airborne for extended periods of time and the role of these small particles has been particularly contentious among influenza researchers.
In a paper published recently in the “Journal of Infection”, Thomas P. Weber and Nikolaos Stilianakis from the JRC Institute for the Security and Protection of the Citizen (ISPC) have challenged the dominant view on influenza transmission by critically reviewing all published information on the ability of the influenza virus to survive outside of its host. The scientists, part of the JRC's Crisis Monitoring and Response Technologies (CriTech) research team, conclude that indoors, influenza virus can remain infectious in fine droplets for up to a few hours. Inhalation of these fine droplets is likely to be a significant factor in transmission because an infectious dose can be very small if the virus is thus directly delivered into the lung. Large droplets, on the other hand, quickly settle on surfaces in the environment unless they are immediately inhaled by persons less than one meter away. Weber and Stilianakis argue that, based on mathematical models and on behavioural data, this is probably very rare. Influenza A viruses such as human flu survive on hands only for a few minutes and on other surfaces slightly longer but spreading them would appear to be possible through repeated contamination of surfaces and hand contact.
The argument that fine aerosol droplets may play a prominent role in transmission could have important implications for preparatory measures against pandemics. During an influenza pandemic against which no vaccine is available, the implementation of other control measures becomes necessary. Surgical masks only stop large droplets, whereas so-called respirators are needed to block fine droplets. The efficiency of such respirators, which are not easy to handle, has never been investigated if they are used in large populations.
The paper, "Inactivation of influenza A viruses in the environment and modes of transmission: A critical review" [10.1016/j.jinf.2008.08.013], accepted 27 August 2008 and online since 9 October is available online here.