Face masks have traditionally been used in general infection control, but their efficacy at the population level in preventing transmission of influenza viruses has not been studied in detail. Data from published clinical studies indicate that the infectivity of influenza A virus is probably very high, so that transmission of infection may involve low doses of virus. At low doses, the relation between dose and the probability of infection is approximately linear, so that the reduction in infection risk is proportional to the reduction in exposure due to particle retention of the mask. A population transmission model was set up to explore the impact of population-wide mask use, allowing estimation of the effects of mask efficacy and coverage (fraction of the population wearing masks) on the basic reproduction number and the infection attack rate. We conclude that population-wide use of face masks could make an important contribution in delaying an influenza pandemic. Mask use also reduces the reproduction number, possibly even to levels sufficient for containing an influenza outbreak.

Influenza virus may be transmitted through the respiratory route by inhalation of an aerosol of non-sedimenting droplets, or by deposition of sedimenting droplets in the upper respiratory tract. Whichever of these is the predominant route for infection with influenza virus has been subject of continuing debate, resulting in detailed studies of aerosol versus droplet exposure. A decisive knowledge gap preventing a satisfying conclusion is absence of a well defined human dose response model for influenza virus. This study uses a hierarchical approach generalizing over twelve human challenge studies collected in a literature search. Distinction is made between aerosol and intranasal inoculation. The results indicate high infectivity via either route, but intranasal inoculation leads to about 20 times lower infectivity than when the virus is delivered in an inhalable aerosol. A scenario study characterizing exposure to airborne virus near a coughing infected person in a room with little ventilation demonstrates that with these dose response models the probabilities of infection by either aerosol or sedimenting droplets are approximately equal. Droplet transmission results in a slightly higher illness risk due to the higher doses involved. Establishing a dose response model for influenza provides a firm basis for studies of interventions reducing exposure to different classes of infectious particles. More studies are needed to clarify the role of different modes of transmission in other settings.