A further scientific peer review of four 2009 Canadian studies confirms that a previous vaccination against seasonal flu possibly increased the risk of infection with pandemic H1N1 (pH1N1) flu. The review is published in the journal PLoS Medicine.
In a school outbreak of pH1N1 in spring 2009, researchers found that people with cough and fever had received prior seasonal flu vaccination more often than those without. As a result, several Canadian public health agencies undertook studies to investigate further. The four studies included approximately 2,700 people with and without pH1N1.
One of the studies confirmed that the 2008 seasonal vaccine provided protection against seasonal influenza, but found it to be associated with an increased risk of approximately 68% for pH1N1 disease. The other three studies similarly found between 1.4-2.5 times increased likelihood of pH1N1 illness in people who had received the seasonal vaccination compared to those who had not. Prior seasonal vaccination, however, was not associated with an increase in hospitalization among those who developed pH1N1 illness.
The studies do not show whether there was a true cause-and-effect relationship between seasonal flu vaccination and subsequent pH1N1 illness (as might occur if, for example, the seasonal vaccine modified the immune response to pH1N1), or whether the observed association was not a result of vaccination, but was instead due to difference in some unidentified factor(s) among the groups being studied.
The researchers note that the World Health Organization has recommended that pH1N1 be included in subsequent seasonal vaccine formulations. This will provide direct protection against pH1N1 and thereby obviate any risk that might have been due to the seasonal vaccine in 2009, which did not include pH1N1.