09/14/2026
When is the best time to get a flu shot? Think pumpkins.
TL;DR:
➡️ Flu protection fades a bit each month after you get the vaccine.
➡️ Getting vaccinated too early might leave you more vulnerable during peak flu season (Dec-Feb).
➡️ But waiting too long could leave you unprotected early in the season.
➡️ Any flu vaccine always beats no flu vaccine, by a wide margin.
➡️ If you want to fine-tune your timing, late October is a
reasonable sweet spot.
(For the full post with links and figures, please read on Substack: https://jenndowd.substack.com/p/when-should-you-get-your-flu-shot)
*Rule #1- Just get your flu shot*
Pharmacies love to start promoting their flu shots as early as August. And from a public health perspective, making vaccines as convenient as possible is smart. If getting your flu shot now means you won’t forget, please... do it! The vaccine is the best way to reduce your risk of severe flu illness.
But for you prone-to-over-analyzing data nerds (you know who you are), waiting until October may give you a slight edge for protection that lasts the whole flu season.
The timing of the flu season varies, but it typically picks up in November and peaks in January-February. Significant flu activity often lingers well into the late Spring.
Observational studies generally show that flu vaccine effectiveness declines about 8-9% each month after vaccination and may wane slightly faster for people over age 65.
This data is consistent with what we know from measuring people’s antibody levels after vaccination. Here we can see the dynamics of a high initial antibody response that gradually tapers off, but stays higher than before vaccination. (This paper actually has cool detail on different types of responders, but that’s a topic for a different day).
Observational studies (just watching what happens in real life, rather than in a clinical trial) are helpful, but are subject to potential biases. For example, if high-risk people are more likely to visit the doctor often and get an early flu shot, this could make it look like early flu shots are less effective, even if they are not.
To get around this, one study used a clever “natural experiment” to look at the timing of flu vaccination in kids (ages 2-5). Because young kids often go to the doctor around their birthday for their annual check-up, kids whose birthdays were earlier in the fall got their flu shot on average a bit earlier. Because a kid’s birth month shouldn’t have anything to do with their underlying health conditions, this mimics an experiment more than standard observational studies.
Using this design, the paper found that kids who got the flu shot in October thanks to their birthday timing had the lowest risk of the medically diagnosed flu compared to those who got vaccinated earlier (or later).
As you can see in the above figure, the difference in flu diagnosis was not huge—amounting to 12% lower odds of flu for kids vaccinated in October compared to August. But some other data has shown that vaccine effectiveness doesn’t wane as quickly in children compared to older adults, so the impact of timing in older folks might be bigger. The real strength of the study is the unique quasi-experimental design. Using month of birth to “quasi-randomly” assign the flu vaccine slightly earlier or later makes us more confident that the timing effect is real, rather than something about the people who choose to get vaccinated early vs. later.
When might earlier vaccination be preferred?
✅ Pregnancy
If you are in your last trimester of pregnancy, your body needs several weeks to make antibodies and transfer them through the placenta, where they will protect your newborn baby after birth. So, if you are due anytime in the autumn, it’s best to get the flu vaccine right away.
✅ Young children
If this is your child’s first flu vaccine, they will need two doses at least four weeks apart to mount full protection. Getting a first dose in August or September works well in this case.
Don’t let the perfect be the enemy of the good.
Some people in public health worry that telling people to wait may mean missed vaccinations….because life gets in the way. I appreciate this point, especially for messaging to big and diverse populations. And I want to emphasize that we have evidence that substantial flu vaccine protection persists for many months. And that any difference in protection from vaccine timing is not large. Not everyone agrees that delaying vaccination is worth it. So getting the flu vaccine now is a good choice.
But I also know that people who subscribe to a newsletter called “Data for Health” might want to optimize their health on the margin where possible. And for older people who are more vulnerable to severe flu, the impact of timing might be bigger. If that’s you, waiting until the Halloween season for your flu vaccine is a reasonable choice based on current data.
Epidemiologist Michael Osterholm has been vocal in recommending timing your flu vaccine more precisely to when flu starts circulating in your local area, even if that is later than October. That strategy requires a level of vigilance in tracking local flu transmission that might challenge even the big data nerds. But I understand the rationale to match peak protection to peak exposure risk and prolong protection as long as possible. If you want to go that route, just remember that antibody protection takes a couple of weeks to build after vaccination, and there is often a lag between deciding to get a vaccine and getting it in your arm. Travel and socializing plans also matter. I always make sure I get my flu shot by a couple weeks before Thanksgiving at the latest, for my own sake and to protect older relatives.
What about timing the COVID vaccine?
At the moment, the timing of COVID waves is less predictable than for flu, and infections happen year-round. So, while COVID protection is also highest in the first couple of months after vaccination before gradually waning, vaccine timing is harder to optimize. COVID has been rising in much of the US, so if you haven’t had a recent infection, now may be a good time to get the updated vaccine. (And yes, you can get it the same time as your flu shot if you want.)
Bottom Line:
✅ Getting your flu vaccine in late October may be the sweet spot for timing.
✅ But an early or late flu vaccine is WAY BETTER than never.
Stay well,
Jenn
This post is a collaboration with Data for Health, Those Nerdy Girls, and the Dartmouth International Vaccine Initiative.
For the full post with links and figures, please read on Substack: https://jenndowd.substack.com/p/when-should-you-get-your-flu-shot
