The nature of vaccines

An influenza vaccine contains three strains (two type A and one type B), representing the flu viruses likely to circulate in the upcoming season.  

The current influenza vaccination formula contains A(H1N1) and A(H3N2) and Type B. The vaccine is made from highly purified, egg-grown viruses that have been made non-infectious (inactivated). Because the vaccine viruses are initially grown in embryonated hens’ eggs, the vaccine might contain small amounts of residual egg protein. Two formulations are available: northern and southern formulations, with the former made available in October and the latter in April of each year.