Unfortunately, we won’t know whether the flu vaccine is a good match to circulating flu viruses until well into the flu season. It’s not possible to predict with certainty if a flu vaccine will be a good match for circulating flu viruses.
A flu vaccine is made to protect against the flu viruses that research and surveillance indicate will likely be most common during the season. However, experts must pick which flu viruses to include in a flu vaccine many months in advance in order for flu vaccines to be produced and delivered on time.
CDC works to characterize about 2000 circulating flu viruses every year to compare them to the virus strains in the vaccine. This helps them understand the flu vaccine’s ability to produce an immune response in people. It also helps them decide what strains to include in the next season’s flu vaccine.