How is the flu vaccine determined in each year?

The influenza viruses in the seasonal flu vaccine are selected each year and the selection is based on surveillance data which indicates which viruses are circulating and also which viruses are the most likely to circulate during the coming season. The degree of similarity between available vaccine viruses and circulating viruses also is important and also the vaccine viruses must be similar to the influenza viruses predicted to circulate most commonly during the upcoming season. Another important thing to look at is whether or not there is a good vaccine virus available the a virus that could be used in vaccine production and which would likely protect against the viruses likely to circulate during the upcoming season.