Seasonal flu shots offer some protection against the H1N1 pandemic virus, a new study suggests, making earlier decisions by most provinces and territories to scale back or suspend plans to deliver the vaccine all the more perplexing.
The study, published online in the British Medical Journal , found that people with laboratory-confirmed swine flu at a hospital in Mexico City were less likely to have received the flu shot in the previous winter compared with patients with other diseases.
It adds one more twist to this fall's flu vaccination campaigns.
Another Canadian study, which is being peer-reviewed, suggested that people are twice as likely to contract pandemic H1N1 if they have received a seasonal flu shot. The research contributed to most provinces and territories changing their vaccination campaigns.
Most have announced they will offer seasonal shots this month to the elderly, who are at low risk of catching H1N1, and residents in long-term care facilities. After the swine flu vaccine campaign is completed, they plan to offer the seasonal flu shot more broadly. Quebec and Nunavut are deferring their seasonal flu campaigns until after their H1N1 mass vaccination clinics. And New Brunswick is going ahead with its original seasonal flu shot campaign before offering pandemic flu shots.
Donald Low, chief microbiologist at Toronto's Mount Sinai Hospital, said the study out of Mexico will add to the confusion among Canadians about which jurisdiction made the right decision. But he cautioned about reading too much into the association between the seasonal flu shot and the H1N1 virus. In medicine, an association is a connection between two events; it does not mean there is necessarily a cause and effect.
"This is a finding, and it's interesting, and maybe very important," he said. "But it has to be substantiated."
The researchers compared the health outcomes of 60 patients with swine flu and 180 patients with other diseases. They believe the reason the seasonal flu shot offers some protection is because it boosts antibodies in people who have exposed to a similar virus either by being infected or through a vaccine.
The authors acknowledge their sample size is small, and the results of their research, as well as the Canadian data, would need further examination.
David Jensen, spokesman for Ontario's Ministry of Health and Long-Term Care, said the province is not reconsidering its vaccination campaign as a result of the Mexican data. He said the Canadian study was not the only reason behind the ministry delaying the seasonal flu vaccine for certain groups. The H1N1 strain is expected to be the main strain circulating this fall, and staggering vaccination programs would provide less of a logistical challenge this season, he added.