# 4473
Last September news of a controversial unpublished Canadian study began to emerge that suggested those who had received a seasonal flu shot in the previous year were more susceptible to the pandemic H1N1 virus.
Helen Branswell, writing for the Canadian Press, was among the first to report on what was soon to be known as the `Canadian Problem’ (see Branswell On The Canadian Flu Shot Controversy).
In short order a number of Canadian Provinces halted or announced delays in rolling out the seasonal flu shot, even though the study had yet to be published (see Ontario Adjusts Vaccination Plan).
The CDC and the World Health Organization, meanwhile, stated they saw no evidence in their data to suggest a link, but that they were continuing to investigate the matter.
As if this weren’t confusing enough, in early October we saw a study published in the BMJ (British Medical Journal) that suggested exactly the opposite - that getting the seasonal flu vaccination may be slightly protective against the swine flu (see When Studies Collide).
This also ran contrary to what we’d heard previously from the CDC, which maintained that the seasonal vaccine was not expected to offer any protection against the novel H1N1 swine flu virus.
Published 6 October 2009, doi:10.1136/bmj.b3928Objective To evaluate the association of 2008-9 seasonal trivalent inactivated vaccine with cases of influenza A/H1N1 during the epidemic in Mexico.
Conclusions Preliminary evidence suggests some protection from the 2008-9 trivalent inactivated vaccine against pandemic influenza A/H1N1 2009, particularly severe forms of the disease, diagnosed in a specialty hospital during the influenza epidemic in Mexico City.
By the first of November, with no corroboration of the `Canadian Problem’, Canada’s National Advisory Committee on Immunization (NACI) came out in favor of resuming seasonal flu jabs (see NACI: Canada Should Resume Seasonal Flu Vaccinations).
Which brings us to today’s study published in the Public Library of Science (PLoS) that looks at this vaccine controversy. As you might have guessed, even after six months, definitive answers are in short supply.
Maryn McKenna, writing for CIDRAP News, brings us the details. Follow the link to read it in its entirety.
New Canadian studies suggest seasonal flu shot increased H1N1 risk
Maryn McKenna
Contributing Writer
Apr 6, 2010 (CIDRAP News) – Despite a rapidly launched range of studies, investigators in Canada are still unable to say—or to rule out—whether receiving a seasonal flu vaccination in the 2008-09 season made it more likely that Canadians would become ill from 2009 pandemic H1N1 flu.
In a lengthy article published today in Public Library of Science Medicine (PLoSMed), researchers detail the results of four supplementary studies that were launched after an April 2009 school outbreak provided the first signal of an association between seasonal flu shots and pandemic flu illness. The studies, which took in about 2,700 people, found overall that the likelihood of needing medical attention for pandemic flu was 1.4 to 2.5 times greater among people who were vaccinated the previous fall.
But the authors warn that, since all four studies were observational, even careful design cannot rule out the possibility that some undetected methodologic bias affected the results. That caution is echoed in a companion editorial, written by US researchers unconnected to the Canadian study, who cite the contradictory results of six other studies conducted in Mexico, Australia, and the United States at the same time as the Canadian ones. Four of those studies found no association between seasonal flu vaccination and pandemic flu illness, while the two done in Mexico paradoxically found that seasonal flu shots may have had a protective effect.
Skowronski DM, De Serres G, Crowcroft NS, et al. Association between the 2008–09 seasonal influenza vaccine and pandemic H1N1 illness during spring–summer 2009: four observational studies from Canada. PLoS Med 2010 Apr 6;7(4) [Full text]
Viboud C, Simonsen L. Does seasonal influenza vaccination increase the risk of illness with the 2009 A/H1N1 pandemic virus? PLoS Med 2010 Apr 6;7(4) [Full text]
See also:
Sep 4, 2009, Canadian Press story "Flu shots increase risk of H1N1: unpublished study"
