One of the most common misconceptions about influenza immunization is the belief that if the vaccine is given early in the fall, say in August or September, it somehow wears off or will not be protective for the whole influenza season. Influenza season typically runs from late December to March or April in the US. Many people, including physicians, believe that people should not be immunized early and it is better to wait until October or November so the vaccine will last through the end of the season.
Like most myths, there is a little bit of truth to this idea, but it is mostly false. When you receive a vaccine, your body, specifically your immune system, remembers that vaccine forever. Unless you have a bone marrow transplant and replace your whole immune system, you will always remember that vaccine and your immune system will protect you. People who survived the 1918 influenza pandemic as children were tested late in life, and their immune system still recognized the 1918 virus.
The little bit of truth to the lack of protection late in the season does not relate to vaccine failure. It is related to the virus. Influenza is a genetically unstable virus; it mutates often and quickly. The problem is that the vaccine protects us against the viruses that were around early in the season, but the virus sometimes changes during the season, so the vaccine may be less protective as time goes on. That is because the virus has changed, not because the vaccine failed.
It is best to be immunized as soon as you can each fall because it takes your body a few weeks to process the vaccine and produce immunity, and you don’t want the virus to infect you before your body is ready. If the virus mutates late in the season, that’s too bad for all of us, but waiting to be immunized would not have helped in that situation.