The Advisory Committee on Immunization Practices (ACIP) of the CDC is in charge of setting the nation’s vaccine policy. If the committee recommends a vaccine, then insurance companies jump on board and pay for it. Interestingly, though, if a new and improved vaccine gets approval, the ACIP is hesitant to state a preference. For example, the relatively new quadrivalent flu vaccine provides protection against 4 strains of flu as compared to protection against 3 strains by the trivalent formulation. Both forms have the same two A strains in them—this year an H3N2 and an H1N1. The quadrivalent has an extra B strain. Does it make a difference?
It will if the circulating B strain is not the one chosen to include in the trivalent vaccine. Influenza B strains fall into one of two lineages, Victoria or Yamagata. A vaccine made from one of these families will offer basically no protection against strains in the other family. In February of every year, the WHO and CDC decide on which strains to put in the next year’s vaccine so that vaccine companies can start gearing up for production. Most years a B strain will circulate along with an A strain. Some years both B strains circulate. And during the H1N1 pandemic, hardly any cases of flu caused by a B strain were detected. If you look at the decade before the quadrivalent vaccine became available, the experts predicted the wrong B strain about 50% of the time. In other words, instead of meeting, they should just have flipped a coin and they would have been right 50% of the time.
So, the question is: the quadrivalent flu vaccine should be, on average, approximately what percentage more effective than the tirvalent?
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