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Seasonal Flu Vaccine Worsens H1N1 Risk?

By Kirsten Houmann, Ivanhoe Health Correspondent

ORLANDO, Fla. (Ivanhoe Newswire) -- Did previous vaccination against seasonal flu increase the risk of getting H1N1 flu? Based on studies by Canadian researchers, the answer is, “possibly.”

In a spring 2009 school outbreak of H1N1, a group of people with cough and fever were found to have received prior seasonal flu vaccination. Several public health agencies in Canada initiated four studies during the summer of 2009 to investigate. Taken together, the four studies included approximately 2,700 people with and without H1N1.

The first study assessed the frequency of prior vaccination with the 2008 seasonal vaccine in people with H1N1 influenza. This study confirmed that the seasonal vaccine provided protection against seasonal influenza, but found it to be associated with an increased risk of approximately 68 percent for H1N1.

The other three studies similarly found approximately one and one-half times increased likelihood of H1N1 illness in people who had received the seasonal vaccine. Prior seasonal vaccination was not associated with an increase in hospitalization among those who developed H1N1.

The studies do not show whether there was a true cause-and-effect relationship between seasonal flu vaccination and subsequent pH1N1 illness, or whether the observed association was not a result of vaccination, but was instead due to differences in some unidentified factors among the groups being studied.

"You can't say that there's a causal relationship here," Ira Longini, Ph.D., of the Vaccine and Infectious Disease Institute at the Fred Hutchinson Cancer Research Center and the University of Washington, Seattle, who is not associated with the study, told Ivanhoe. "You can simply say there's potential association between the increased risk of infection and illness and to pandemic influenza as a function of seasonal vaccination. This might warrant further investigation, but that's as far as it goes."

If the findings from these studies are accurate, they raise important questions about the biological interactions between pre-existing and novel pandemic influenza strains, researchers say.

Study authors note that the World Health Organization has recommended that H1N1 be included in subsequent seasonal vaccine formulations. This will provide direct protection against H1N1 and thereby obviate any risk that might have been due to the seasonal vaccine in 2009, which did not include H1N1.

Dr. Longini says the best solution is a trivalent vaccine, which works against the influenza B, H1N1 and seasonal H3N2 viruses.

SOURCE: Ivanhoe interview with Ira Longini, Ph.D.; PLoS Medicine, April 6, 2010