How Well Do Flu Vaccines Work?
It depends on how closely the flu viruses used in the vaccine match the actual flu virus that hits a population and the immune system of the vaccinated person. Flu vaccines change every year. Since the flu virus changes each year, the flu vaccine changes as well. There are many strains of flu and it is difficult to tell which one will circulate in any given year. Moreover, because it can take up to nine months to manufacture a flu vaccine, vaccine producers have to predict early in the year which influenza virus will circulate in the following year. To do this, vaccine makers collect flu viruses from all around the world every year to decide which viruses should be used. The three strains judged most likely to cause trouble are selected (two Type A strains and one Type B). How well the vaccine works in any given year depends on how closely the three vaccine strains match the actual flu viruses that eventually circulate. Flu vaccines protect people from becoming sick. Even when the vacc
