Flu shots are vaccines against the most common strains of the flu virus. most vaccines are rarely changed but new flu vaccines are produced every year. why are flu shots constantly being reformulated? the flu virus is not completely known.

Scientists are creating vaccinations that prevent people from getting bird flu. If the virus mutates, a new vaccine needs to be made. The vaccine needs to be tested, which can be dangerous. It also takes a long time for scientists to know whether the vaccine works

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