Background: Annual epidemics of influenza are the most important cause of acute respiratory illnesses requiring medical attention. The current novel influenza A (H1N1) pandemic raises further concerns about influenza transmission. The infection can be transmitted through inhalation of respiratory droplets from coughing or sneezing. The size of the infectious particles ranges from approximately 0.1 to 100 μm, and it is not known what role this range of particles plays in transmission. There are few comparative studies of respiratory protective devices to prevent inhalation of these particles. Especially during pandemics, the N95 respirators are likely to be in short supply and unavailable in many countries. Loeb and colleagues compared the effectiveness of surgical masks and N95 respirators in preventing influenza transmission among acute-care nurses.

The Study: Nurses working in emergency departments and medical and pediatric units in eight acute-care hospitals in Ontario were recruited for this randomized trial. Nurses were eligible if they expected to work full time on study units during the 2008–2009 influenza season, and had current fit-test certification. Investigators were blinded to the randomization process, although it was not possible to alter the masks to make them indistinguishable. At the start of the influenza season, nurses began using the respirators or surgical masks in addition to gowns and gloves for any contact with patients who had febrile respiratory illness. The study was designed to continue through the end of the influenza season but was stopped on April 23, 2009, when the Ontario Ministry of Health and Long-Term Care recommended that all health care workers use the N95 respirators when caring for patients with influenza-like illnesses. Participants had serologies against influenza drawn at study entry and at the end of follow-up; they were also instructed to use nasopharyngeal swab techniques if they developed any influenza-like symptoms. Participants were assessed twice weekly with Web-based questionnaires, and an additional follow-up contact was made if the questionnaires were not completed. The primary outcome was the incidence of laboratory-confirmed influenza.

Results: Between September 23 and December 8, 2008, a total of 446 nurses were enrolled in the study, with 225 randomly assigned to the surgical mask group and 221 assigned to the N95 respirator group. The demographics in each groups were similar, including equally low rates of seasonal influenza vaccination (30.2 percent in the surgical mask group, 28.1 percent in the N95 respirator group). Follow-up started January 12, 2009, and ended on April 23, 2009. There were 50 cases of laboratory-confirmed influenza in the surgical mask group (23.6 percent) and 48 in the N95 respirator group (22.9 percent), indicating that the mask is not inferior to the respirator in protecting against influenza. The study could not determine if the affected nurses acquired influenza in the hospital or in the community.

Conclusion: In a head-to-head comparison, surgical masks appear to be as effective as N95 respirators in preventing influenza transmission in health care workers.