1. Do respirators offer any
additional protection against
influenza over surgical masks?
John W Cherrie, Laura MacCalman and C Lang Tran
Conclusions The boxes in the Figure are represented as states in a discrete-time
Markov-Chain model through which, given initial pathogen loads and
This analysis suggests that at most wearing a calculation of probabilities and transfer rates, the risk of infection of an
respirator would reduce infection risk by about attending health care worker were estimated for various conditions.
60% and a surgical mask would at most reduce Results and Discussion
risk by about 25% The predicted difference in
This modeling approach suggests that between 27% and 93% of
protection between respirators and surgical masks
infection risk may be due to hand contact, between 3% and 58% from
was relatively small. droplet spray and between 1% and 25% from respirable/inhalable
aerosol.
Introduction
Laboratory studies have demonstrated that respirators are effective at
It is difficult to attribute the relative contribution to infection risk of reducing exposure to aerosols (possibly up to 100 fold reduction) and
different routes of exposure to virus particles. However, in a specific surgical masks provide some protection against droplet spray (about a
scenario, for example caring for a patient in a hospital room, it is five fold reduction in exposure). Using these data it is possible to
possible to use mathematical modeling to estimate the risk from inhaling estimate the effect of facemasks on risk – in this case it suggests that at
respirable/inhalable droplets containing virus particles, from contact with most wearing a respirator would reduce infection risk by about 60% and
large droplets from coughs or sneezes (droplet spray) and by transfer of a surgical mask would at most reduce risk by about 25%
virus particles to the mouth or mucous membranes by hands or objects.
The model
We have devised a simple conceptual model of these processes based on
Nicas and Sun (2006).
The graph shows the predicted reduction in risk from wearing a mask in
relation to saliva virus concentration (upper lines respirator, lower lines
facemask – yellow and red points represent different assumptions about
relative infectivity by route). The difference in protection between
respirators and surgical masks was relatively small.
References
Nicas and Sun. An integrated model of infection risk in a health-care
The model comprises a source, a series of compartments connecting environment. Risk Anal (2006) vol. 26 (4) pp. 1085-96.
pathways for the exchange of virus particles (represented by the Nicas and Jones. Relative contributions of four exposure pathways to
lines). Loss of viability is also accounted for in the model scheme. influenza infection risk. Risk Analysis (2009) vol. 29 (9) pp. 1292-1303.
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