Most people know that the reason to get a flu shot every year is because one year’s vaccination does not necessarily protect against the next year’s flu virus.
That’s because the flu virus is terrifically clever. Viruses want to live. And to live they need to replicate themselves. Replication requires that the virus take over a cell and use the material in that cell to make copies of itself. Vaccines target foreign material in the cells and stimulate antibodies (specialized proteins) to destroy the virus or other pathogen. In many diseases, for example, hepatitis or measles, the virus doesn’t change; therefore the vaccine against these diseases is effective for years and years. However, the flu virus changes, or mutates, so that it tricks the immune system into not recognizing it and therefore not fighting it.
Changing is the virus’s survival strategy. If the immune system recognizes even a slight change (mutation), it won’t react to the virus as an intruder and therefore antibodies won’t develop. This is evolutionarily sound for viruses: small changes help the virus survive and multiply. Our vaccines can’t keep up with the changes.
Given how this works, it’s remarkable that a vaccine has any benefit at all. Even though one step behind, it prevents thousands of deaths each year and cuts the chance of getting the flu by half. The World Health Organization meets every year in February to try and figure out which flu strains will infect the most people in the coming year. Once identified, manufacturers of the vaccine have to quickly make the vaccine and distribute it. Some years the vaccine has greater protection powers than other years. Also, the vaccine works better in some people because they may have a stronger or weaker immune response.
Nonetheless getting the flu vaccine means that your cells and antibodies have some experience with the virus, or parts of it, and even when not perfectly matched, it may save you from severe illness, hospitalization, and even death. It’s wise to get a yearly flu shot.