Which type of flu vaccine is most effective?

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In a year with poor match-up between influenza strains as found in the vaccines, is there any advantage to one type of vaccine over another with regard to efficacy?—ANN THOMPSON, PA-C, MPAS, Omaha

There are two types of vaccine for the prevention of seasonal influenza: inactivated (flu shot) and live attenuated vaccine (nasal spray). The same strains are in both vaccines. The nasal spray does contain live viruses, but they are weakened and cold-adapted, which means they cause infection only in the nose and not in the lungs (where warmer temperatures exist). The live attenuated vaccine is approved for use in healthy people aged 2 to 49 years. This vaccine should not be given to pregnant women. The efficacy of influenza vaccines vary from year to year and may differ for inactivated and live vaccines. A randomized, double-blind, placebo-controlled trial of licensed inactivated and live attenuated influenza vaccines in healthy adults during the 2007-2008 influenza season found the inactivated vaccine to be 50% more efficacious than the live attenuated vaccine in the adult population in which both vaccines are approved for use (N Engl J Med. 2009;361:1260-1267, available at content.nejm.org/cgi/content/full/361/13/1260, accessed June 17, 2010). It was speculated that this difference was related to the inability of the live attenuated viruses to infect some adults because of their past exposure to similar strains. This situation might be different in years in which there is major antigenic drift, which was not the case during the time of the study.—JoAnn Deasy, PA-C, MPH (141-2)

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