September is not too early to get a flu shot. It’s actually one of the two best months to get vaccinated. The CDC recommends September and October as the ideal window for most people to receive their annual flu vaccine.
Why September Hits the Sweet Spot
Flu vaccine protection doesn’t last forever. It gradually weakens over the months following your shot. A Kaiser Permanente study found that the odds of testing positive for flu increased by roughly 16% for every additional 28 days after vaccination. By five months out, vaccinated people had about twice the odds of testing positive compared to those who were recently vaccinated.
This is why timing matters. Get vaccinated too early (July or August), and your protection may fade before flu season ends. Wait too long past October, and you risk catching the flu before your immune system has time to respond to the vaccine, which takes about two weeks. September strikes the right balance: your immunity builds in time for early flu activity and holds strong through the worst of the season.
In the United States, flu activity peaks between December and February. Looking at more than 40 flu seasons of data, February has been the peak month most often (18 seasons), followed by December (9 seasons) and January (6 seasons). A September vaccination gives you solid protection across that entire stretch.
July and August Are Too Early for Most People
The CDC specifically advises against July and August vaccination for most adults. This is especially true for adults 65 and older, whose immune response tends to wane faster. Pregnant people in their first or second trimester should also avoid those early months unless they won’t be able to get vaccinated later.
There are a few exceptions. People in the third trimester of pregnancy can reasonably get vaccinated in July or August if the vaccine is available, because doing so can pass protective antibodies to the baby during the first months of life, before the infant is old enough to be vaccinated. And anyone who simply won’t have access to vaccination later in the fall should go ahead and get the shot when they can. A slightly early flu shot is always better than no flu shot.
Children Who Need Two Doses Should Start Early
Some young children actually benefit from a September start. Kids between 6 months and 8 years old who have never received a flu vaccine (or whose vaccination history is unknown) need two doses, spaced at least four weeks apart. The same applies to children in that age range who haven’t previously received at least two doses in prior seasons. Starting the first dose in September ensures the second dose is on board well before flu starts circulating heavily.
Getting Your Flu and COVID Vaccines Together
If you’re also due for an updated COVID-19 vaccine this fall, you can get both shots at the same visit. There’s no required waiting period between the two. Studies have shown that people who received both at once were slightly more likely to experience fatigue, headache, and muscle aches compared to getting just the COVID vaccine alone, but those side effects were mostly mild and resolved quickly.
If you’d rather space them out, that’s fine too. There’s no minimum waiting period between the two vaccines. The practical advantage of getting both at once is simply convenience: one trip, both arms protected. If given in the same arm, the injection sites should be at least one inch apart.
The Ideal Timeline at a Glance
- July or August: Too early for most people. Protection may fade before the season ends. Exceptions include late-pregnancy vaccinations and people who can’t return later.
- September or October: The recommended window. Aim to be vaccinated by the end of October.
- November or later: Still worthwhile. Flu can circulate into March or April, so getting vaccinated after October still provides meaningful protection.
If you’re seeing flu vaccines available at your pharmacy or doctor’s office in September, that’s your signal. The timing is right.