What do we know about the research?

An unpublished Canadian study has abruptly modified vaccination programs across the country. Most provinces have announced that they will delay their seasonal flu-shot programs to most residents under the age of 65. Nunavut and Quebec have said they would put off all seasonal flu shots until after Christmas, in favour of rolling out the H1N1 vaccine first. New Brunswick has taken a different route: It's offering all its residents the seasonal flu shot next month, followed by the H1N1 vaccine.

Who are the authors of this study?

The research is led by Danuta Skowronski, an epidemiologist with the British Columbia Centre for Disease Control, and Gaston De Serres of Laval University.

What does the study say?

The researchers studied data from about 1,500 Canadians in B.C., Alberta, Ontario and Quebec, after receiving word of a possible association between the seasonal flu vaccine and the risk of H1N1 in a northern B.C. community. In medicine, an association refers to a connection between two events; it does not mean there is necessarily a cause and effect.

They found that those under 50 years of age are at a higher risk of being infected with the swine flu virus after receiving the annual flu shot. The study also found that despite an increase in the risk of being infected with H1N1, the cases seen were mild disease.

The study included few people with chronic conditions, which ruled out a link that would show only those with underlying health conditions are susceptible.

Do others concur?

Researchers in the United States, Australia and Britain said they have looked for, but haven't seen, the same association.

Where is the study at this point?

The study is undergoing a peer-review process and being considered for an unnamed scientific journal. The Public Health Agency of Canada also set up an independent panel to review the findings. David Butler-Jones, Canada's chief public-health officer, said those findings could be released as early as next week.

Is there more on the puzzling association between the flu shot and the H1N1 virus?

Preliminary research undertaken by the Public Health Agency of Canada on Canadian swine flu cases that end up in hospital suggests that people who get the seasonal flu shot are not at increased risk of being admitted to hospital.