The protection provided by a flu shot typically lasts for about six months. The flu vaccine is designed to protect against the most common strains of the influenza virus for each flu season. For most people, the flu shot offers the highest level of protection within the first two to three months after vaccination, but the immunity gradually diminishes over time. For this reason, annual flu shots are recommended, especially for those who are most at risk, such as older adults, children, pregnant women, and individuals with compromised immune systems.

The flu virus can also change from year to year due to its ability to undergo antigenic drift, which is a process where small changes in the virus’s surface proteins make it harder for the immune system to recognize and fight the virus. That’s why it’s essential to get the flu shot every year, as each year’s vaccine is formulated to protect against the most current strains of the virus. Even if you are vaccinated, it is still possible to catch a flu strain that wasn't included in the vaccine, but the flu shot may reduce the severity of illness or the risk of complications. For optimal protection, it is recommended to get the flu shot in the early fall before the flu season peaks.