Why Your Flu Shot is Only Good for a Year

Most vaccinations confer long-term, even lifetime, immunity. Aside from a periodic booster shot a few times during one’s life, vaccinations against diseases like tetanus and measles never need to be repeated. Influenza vaccinations are different. For many people, getting a flu shot in the arm is an annual ritual. Why is your flu shot only good for a year?

The adaptive immune system has a remarkable capacity to learn. Once exposed to a given antigen – such as a protein from an influenza virus strain – certain immune system cells called memory B cells "remember" that antigen for the rest of the individual's life. But influenza mutates so rapidly that it can become unrecognizable to B cells that remember a previous version of the virus.

In fact, just like other vaccinations, the influenza shot confers long-term immunity. The repeated vaccinations are needed because there is not just one virus that causes influenza, but many. Influenza is a rapidly changing virus, and there are countless strains, only a few of which are dominant at any given time. Both Influenza A and Influenza B, the two most serious species of flu virus, mutate at a steady clip through a process called antigenic drift. Antigenic drift refers to changes in the surface proteins of the virus particle that affect how the immune system recognizes the virus.

The flu shot is intended to protect against “seasonal influenza.” Seasonal flu is not a specific disease, but rather part of the worldwide pattern (epidemiology) of influenza. In both the northern and southern hemispheres, influenza cases naturally increase during the colder months – starting around November in the north and May in the south. The influenza strains from one hemisphere will be the ancestor strains for the influenza strains for the other hemisphere half a year later.

Based on which strains are affecting people during a given flu season, as well as an understanding of the general patterns of flu virus evolution and spread, scientists make predictions about which strains are likely to dominate the other hemisphere’s upcoming flu season. A flu vaccine is produced for the likeliest candidates; ideally, the vaccine will teach the memory B cells to recognize the coming influenza viruses, making vaccinated people immune.

One year later, or even many years later, those people will still be immune to the influenza virus strains from the vaccination. After a year of evolution, however, the next year's strains are likely to be so different that previous vaccinations won't help the immune system recognize them. Superfast influenza evolution is the reason your flu shot is only good for one year.