The effectiveness of the influenza vaccine varies from year to year. Each year the vaccine contains only the three strains of influenza that researchers feel are mostly likely to be circulating in the upcoming flu season. The decision on which strains to include has to be made months in advance in order to produce the millions of doses required. Sometimes the experts make a wrong choice, and the viral strains in the vaccine do not exactly match the strains in the community. Thus, the effectiveness of the vaccine can vary from 50 percent to over 95 percent depending on the match.