When should I get my flu shot? Here’s what science says.

Originally published October 9, 2024. Updated October 15, 2026.

Pharmacies and doctors start booking flu vaccine appointments in late August, long before the number of flu cases are on the rise. The CDC recommends getting vaccinated in September or October. But most years, flu cases don’t peak for a couple months. So what’s the data supporting these recommendations?

How long does the flu vaccine last?

First, we need to consider how long the flu vaccine protects you from the flu. If you get the vaccine too early (ex: August), you could be less protected towards the end of the flu season. The CDC says that if possible, avoid getting vaccinated in July or August (though there are cases where this can be considered). On the flip side, if you wait and get the vaccine too late, you could catch the flu before you get the vaccine or before your body has enough time to generate antibodies to provide protection.

You can still run into this risk by waiting until October. My son’s school actually ended up having three flu cases in the last week of September and first week of October. This was before our family planned to get the flu shots this year. So far, we have made it through unscathed. By now (mid October), we’ve gotten our vaccinations so we’re just waiting for our immune systems to respond.

Over the years, there have been multiple studies on how long the flu vaccine is effective in different populations. We’ll just highlight a couple here.

A study in children aged 2-5 found that October is the optimal time for children to get the flu vaccine. A different study looked at vaccine effectiveness in those 9 years and older using data from the 2011-2012 to the 2014-15 flu seasons. This study found that vaccine effectiveness was highest shortly after vaccination but then declined about 6-11% per month. Another study published in August of 2026 looked at vaccine effectiveness during the 2024-2025 flu season. They saw that vaccine effectiveness against influenza A was highest 2-8 weeks after vaccination before it started to decline. However, they noted that vaccine effectiveness against influenza B was high for the entire season. Vaccine effectiveness can vary from season to season however.

The idea is that getting vaccinated in September or October provides some level of immunity throughout the entire flu season.

Referencing historic flu data

Out of curiosity, I’ve been looking at the influenza tracker in my state (Massachusetts) and the CDC national flu tracker. Often these maps and dashboards can overlay this year’s data with previous years’ data so you can see how things track compared to previous years and some overall trends. This can help you get an idea of when to get the flu shot before cases start to rise.

However, historic data is not always predictive of what will happen this flu season. This year, for example, flu has started rising earlier than last year in the west coast of the U.S. So if you’re taking a look at historic flu data, it can help you see when flu cases start to rise, but know what happens this flu season might be different than years past.

Consider local rates of flu transmission

Flu transmission rates across regions can differ dramatically. In the subtropics and tropics, influenza transmission occurs year round, sometimes producing multiple peaks in transmission. One study suggests the possibility of a semi-annual vaccine for older adults.

Here in the US, flu season typically runs from October to April. Of course, the onset of flu season can differ from year to year, which is why it could be helpful to look at current rates of transmission. My state’s dashboard (as well as the CDC dashboards) includes metrics for current influenza transmission with ratings for activity levels (ex: minimal, low, moderate, high), which can be helpful to interpret what the percentages and numbers mean.

If you’re traveling, consider looking for any flu reporting dashboard for that area. This year, if you’re traveling to the west coast of the U.S. where flu test positivity rates are higher, vaccination two weeks before travel will help.

How long does it take after vaccination before I’m protected from the flu?

While looking at current transmission data is helpful, it takes two weeks after vaccination to generate antibodies against the virus so you’ll have to anticipate what transmission will be like two weeks after you get the vaccine.

For kids receiving their first influenza vaccine, it’ll take a bit longer before they are fully protected. Children (under 9) who receive the flu vaccine for the first time require two doses spaced at least one month apart so if you plan for your child to be fully protected by November, they’ll have to get their second shot in mid October and their first shot before mid September.

TL;DR

The CDC recommends getting your flu shot in September or October. Check out the CDC’s flu tracker or your state’s flu tracker to see why.

I hope this information is helpful! And if you’re reading this past October and haven’t gotten vaccinated yet, it’s not too late! Most flu activity peaks between December and February. Getting the flu shot late still means you are protected for some portion of the flu season.

Want more info on the flu? Check out these blog posts:

• Imagine tasting your flu infection before the symptoms hit
• Move over flu season, there’s an entire epidemic calendar
• The Snotty Science Behind Daycare Respiratory Illness Transmission
• Shorter, milder colds? Iota-carrageenan cuts length and severity of upper respiratory infection symptoms

Medical disclaimer: This website does not provide medical advice. It is intended for informational purposes only. No material on this site is intended to be a substitute for professional medical advice, diagnosis, or treatment.

Featured image: Influenza B viral particles. Source: NIAID/NIH.