Sunnybrook Health Sciences Centre will require doctors, nurses and other hospital staff and volunteers to either get a flu shot or don a mask during influenza season, according to a memo to employees obtained by The Globe and Mail.

Sunnybrook is introducing the new policy this fall in an attempt to control the spread of a virulent and potentially fatal illness that afflicts thousands of Canadians. About 3,500 people die every year from the highly contagious infectious disease, according to the Public Health Agency of Canada.

The new policy is an acknowledgement that voluntary immunization measures are not that effective. Only one of every two health-care workers gets an annual flu shot, even though vaccination is one of the best methods of preventing the spread of influenza. The vaccine has been shown to prevent illness in about 70 per cent of healthy children and adults, according to medical studies.

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"It just seems to be the responsible thing to do to have our staff who are caring for sick and vulnerable people to be innoculated against influenza." Sunnybrook spokesman Craig DuHamel said.

Sunnybrook is introducing a vaccinate-or-mask directive along with other major teaching hospitals in Toronto, says the memo sent to employees on Thursday. The hospitals will not make vaccinations mandatory for health-care workers. Rather, they will offer employees the choice of either being vaccinated or wearing a mask during flu season. Such policies result in substantial increases in vaccination rates among health-care workers, according to a recent Canadian Medical Association Journal article.

Sunnybrook is far from the first hospital to put in place a vaccinate-or-mask policy. Hospitals and long-term care homes in British Columbia, as well as 13 hospitals in Ontario, have adopted such measures. Still, the decision is expected to provoke a backlash from the Ontario Nurses' Association, the union that represents 60,000 nurses and other health-care workers in the province. The ONA has threatened to file grievances against other hospitals that have forced unvaccinated nurses to wear masks.

"Receiving the influenza vaccine needs to be a choice and should be part of a more comprehensive, evidence-based infection control plan," Linda Haslam-Stroud, president of the ONA, said in a statement. "Outing nurses that do not choose vaccination with mandatory mask-wearing does nothing but provide our patients with a false sense of security."

The Registered Nurses' Association of Ontario, which spearheads professional development and political lobbying on behalf of nurses, disagrees with the ONA's stance, saying hospitals have a responsibility to do everything possible to protect patients, including forcing unvaccinated staff and visitors to put on masks. "We are in support of it. Simple as that," said Doris Grinspun, the chief executive officer of RNAO. "The only thing we would say is that it has to apply to everybody, not only the nurses, of course, [but to] all the health professionals and to visitors."

Ms. Grinspun said that even among nurses, unfounded concerns about the safety of the flu shot persist. "I think that there are still a lot of myths of, 'Oh, I got the flu shot today and now I got the flu.' I think that's [true] for the public and also for health professionals," she said. "To me that's an issue of education."