Why does the flu vaccine only last a short amout of time?

Answer 1

It is not so much that the vaccine's effectiveness doesn't last long, it is more related to the ability of the virus to mutate to a different form that the vaccine was not designed to match.

The seasonal flu vaccine is given annually because the common strains of influenza viruses that circulate each flu season are rapidly mutating viruses. Once they mutate, the vaccine we got previously may no longer match the new mutation well enough so that the antibodies we developed after the last vaccine can no longer match well enough to "lock" the new mutated virus from being able to enter our cells.

There are quite a few different viruses that may be among those that "make the rounds" from year to year. Immunologists and epidemiologists in the US work each year to identify which strains are most active on the other side of the globe (and therefore are likely to be the ones that will spread into our side when our season arrives). They usually add three types to the vaccine, typically one of the Type A flu viruses, one of the Type B flu viruses and whatever third type seems to be a good choice based on what is circulating in Asia and the Southern Hemisphere during the summer months to prepare the vaccine for the next fall flu season. It works the same from the southern hemisphere when they have summer and are getting vaccines ready for when they will have their next flu season.

However, if the viruses did not mutate, we would have life long immunity to each one we get vaccinated against. There have been studies that have shown that flu virus vaccines can be effective against the same strains of flu up to 90 years or "life time", unlike some of the other virus or bacterial vaccinations that "wear off" in time, like tetanus or whooping cough.