The traditional seasonal flu vaccine may have increased the risk of infection with pandemic H1N1 swine flu, according to the results of four new studies by Canadian researchers, published in PLOS Medicine.
If accurate (and now there are at least 6 studies that agree with this conclusion), this is not an anomaly. Licensed vaccines have at times been shown to increase, paradoxically, the infections they are meant to prevent. (Then the license is withdrawn.) For details, see my prior post on this topic. According to a detailed article in CIDRAP:
If that finding is not the result of chance or bias that occurred despite careful study design, the researchers say, then several hypotheses might explain the increased risk. Immunization might block the cross-protective immunity created by an earlier infection. “Original antigenic sin,” or immunologic memory dating back to someone’s first flu infection, might create antibody responses that are cross-reactive to other flu strains, but not strong enough to be cross-protective, and thus do not neutralize the virus. Or a related phenomenon called “antibody-dependent enhancement” might lead antibodies to inadvertently enhance virus uptake and production.
Immunity is not a simple thing. Vaccine theory is in its infancy, and vaccine development is based on trial and error.
Here then may be a rather serious unintended consequence of flu vaccination: