How do you know if it is too late to get a flu shot?

Fall is usually considered the beginning of flu season—that is, when the cases of flu start increasing across the country. It’s a time when you’re likely to hear healthcare providers and public health experts urging you to get a flu vaccine by the end of October. If you don’t get vaccinated by then, you may wonder: Is it too late to get a flu shot?

Taking a look at flu activity patterns can help answer this question.

When to get a flu shot: When flu is circulating

The basic rule of thumb about when to get a flu shot is: While September and October are the best times for most people to get vaccinated, the Centers for Disease Control and Prevention (CDC) continues to recommend vaccination as long as flu viruses are in circulation.

Depending on the flu season, flu cases usually begin to rise in October and reach their highest levels between December and February. The flu season can last into April and may even go into May.

Flu activity can vary, depending on the flu season. In the 2023–2024 flu season, flu activity started to rise earlier than in many past flu seasons (although not as early as in the 2022–2023 flu season).

Whenever you decide to get a flu shot, you need to give your body enough time to develop immunity to the flu. It takes about two weeks for your immune system to create enough antibodies to help protect you from the flu.


If you’re considering getting a flu shot later in the flu season but are worried that it’s too late to get a flu shot—if there’s any flu activity occurring, then it’s not too late at all. Speak to your healthcare provider who will know whether flu is still circulating in your community and whether you should get the flu vaccine.

Regardless of the time of year, the flu can cause serious illness and complications that result in hospitalization, or even death. Flu shots are the best way to protect you and others from the flu. If you get a flu vaccine and still get the flu, your flu vaccination can help reduce the severity of illness and the risk for serious flu-related complications.

Learn what kind of flu vaccine is right for you