If a symptom was reported by a participant, a nasal specimen was obtained and tested for the influenza and other respiratory viruses. Influenza infection occurred in 50 nurses (23.6 per cent) in the surgical mask group and in 48 (22.9 per cent) in the N95 respirator group.

The researchers concluded that in routine health-care settings, particularly where the availability of N95 respirators is limited, surgical masks are as effective in protecting against influenza.

"It's certainly good to know that the protective effect of a surgical mark appears to be, based on our data, similar to the N95," said Loeb, an infectious disease physician and microbiologist who is recognized nationally and internationally for his research on SARS, West Nile Virus and influenza.

"This is certainly valuable information for health-care workers in North America who are required to wear N95 masks, but also for those in countries where respirators may not be available."

Following the outbreak of SARS in 2003, the Ontario Ministry of Health and Long-Term Care mandated the use of surgical masks for all health-care workers providing care to or when within one metre of a patient with influenza-like illness.

In April 2009, the ministry upgraded its guidelines on personal protective equipment for health-care workers. It now recommends that N95 respirators be used by health-care workers when within two metres of caring for symptomatic patients.

Source: McMaster University