Surgical masks versus respirators for flu protection

We only just got to the surgical/N95 mask article in the Journal of the American Medical Association (JAMA). We’ve been traveling and haven’t been able to keep up with what others were saying, but we’re sure it’s been well covered by the usual suspects. So we’ll just add our take here, for what it’s worth.

As most readers here know, what kind of mask (if any) will best protect a health care worker or anyone else at high risk of exposure to people infected with influenza virus is a difficult question. We still remain unsure whether flu is transmitted mainly by large or small droplets. If most transmission is by very large droplets produced by coughs and sneezes then a surgical mask might prove effective, but if it is mainly very small particles that remain suspended for long periods and have no difficulty finding their way around a loose fitting surgical mask it’s another matter. For that you want a respirator with a good seal. This means having a proper fit and knowing how to don it to make the seal. If you have facial hair (beard or mustache) you won’t be able to fit one of these respirators. They are also more expensive and not in great supply. So the question is great practical importance.

So how would you find out ? You could try to conduct an experiment with two groups of health care workers, each wearing one kind of mask during the flu season and check to see if there is a difference in the incidence of flu infections. This sounds straightforward, but in fact such studies are quite difficult to do. The study reported in JAMA is a version of this kind of experiment involving 446 nurses in emergency departments, medical units, and pediatric units in 8 large hospitals in Ontario, Canada. This is a good place to do this kind of study because it was where many health care workers caught SARS in 2003 so there is a high sensitivity to the problem. SARS is a disease most contagious in the latter part of the illness, so health care workers were at special risk. Influenza is most contagious in the period shortly after symptom onset, so many hospitalized patients may not be at maximum contagion. The epidemiology is different.

So how was the study done ? Nurses who worked full time in specified units likely to see flu patients were enrolled and randomly assigned to either wear a surgical mask or an N95 respirator whenever they cared for a patient with a respiratory disease with fever at any time during the 2008 - 2009 flu season (enrollments occurred between September and early December and the nurses were followed until the end of April when all switched to N95 respirators because of the onset of the swine flu pandemic). Audits of a small sample showed extremely good compliance. Post-SARS era nurses in the Toronto area take this seriously. Gloves and gowns were worn by both groups, a routine practice in these hospitals.

The type of mask was the variable being assessed against its ability to protect against influenza infection. The nurses were queried for symptoms of influenza-like illness twice weekly via a web-based system. When indicated, the nurse self-swabbed her or his nasopharynx via a specific protocol and these were tested for a variety of respiratory viruses via PCR. Pre-study blood sera was also collected and evaluated for a 4-fold rise in antibody titer against pandemic and seasona flu types and subtypes at the end of the study to verify clinical or occult infection. So what were the results ?

One of the first things you do in a study of this type is to check to see if the two groups are roughly comparable. Randomization can fail in a variety of ways (including differential drop out of participants) but all of the usual measures showed an excellent comparability of the two groups (mean age of about 36 years old and 94% female). About 30% of each group had received seasonal flu vaccine. Obviously this is an important thing to check and if it turned out that there was a significant difference in proportion vaccinated, this would have to be taken into account. But there was very little difference (28.1% for the N95 group, 30.2% for the surgical mask group). What about lab confirmed influenza ?

The figure for the surgical mask group was 23.6% and the N95 group 22.9%. These numbers are about as close as you can get, indicating no difference in outcome, at least by this measure. While there wasn’t any more infection in the surgical mask group, they did experience infection with fever slightly more often (5.66% versus .9%), although the numbers are quite small. In the nurses diagnosed by serology, 65.9% of the surgical mask group were asymptomatic while the number was 70.5% for the N95 group. One wonders, though, whether these findings might be a reflection of size of the viral inoculation, with N95s being more protective in that sense.

The design does not permit differentiating workplace and community acquisition of infection, nor is this study specific to swine flu, although it includes swine flu. It does strongly suggest but not prove that for this setting small aerosols weren’t dominating transmission. If it turns out to be true, that’s good news on several fronts. It makes it easier to protect health care workers from patients, patients from health care workers and all the rest of us can concentrate on the sneezers and coughers in our vicinity rather than the ones that were across the room the day before. And it adds emphasis to cough and sneeze hygiene.

This is not the end of the story, I am sure. But these are some of the best data points we have, so far.