If you’re thinking about getting a flu shot in July or August, it’s probably too early for most people. September and October are the best months to get vaccinated, with the goal of being fully protected by the end of October. The reason comes down to timing: flu vaccine protection fades over time, and getting vaccinated too early can leave you less protected during the months when flu is most likely to hit.
Why Getting Vaccinated Too Early Is a Problem
Flu vaccine protection starts to decline about 41 days after you receive the shot. After that point, effectiveness drops by roughly 9% every 28 days in adults. That decline adds up fast. If you get vaccinated in July, by the time flu peaks in January or February, you could be six or seven months out from your shot, with substantially less protection than someone who got vaccinated in October.
This waning effect is even steeper in older adults. Studies show protection can decline by 10 to 11% per month in people 65 and older, which is why the CDC specifically advises most adults, especially seniors, against getting vaccinated in July or August.
When Flu Actually Peaks
Flu season runs roughly from October through May, but activity peaks much later than most people assume. Looking at more than 40 flu seasons of data, February has been the peak month more often than any other, accounting for 18 out of 42 seasons. December and January are the next most common peak months. March has also been the peak in six seasons.
This means your protection needs to hold up through at least February, and ideally into March. A shot in late September or October lines up well with that window. A shot in July does not.
The Sweet Spot: September and October
The CDC recommends September and October as the ideal vaccination window for most people. Getting vaccinated in this range means your immune response will still be strong during the December through February peak, with enough residual protection to carry you into March if the season runs late.
A study of 800,000 children at Harvard found a clear illustration of this timing effect. Children born in October, who typically received their flu vaccines during routine October checkups, had the lowest flu diagnosis rates that season: just 2.7%, compared to 3.0% among those vaccinated in August or January. That gap may sound small, but scaled across a population, it reflects real differences in how well the timing of vaccination matches the timing of exposure.
The goal is to be vaccinated by the end of October. It takes about two weeks after vaccination for your body to build full protection, so a late October shot still has you covered before most flu activity ramps up in November and December.
Exceptions Worth Knowing About
A few groups have good reasons to get vaccinated earlier than September.
- Pregnant women in their third trimester: If you’re in your third trimester during July or August and the vaccine is available, early vaccination is reasonable. Getting the shot during pregnancy passes protective antibodies to your baby, shielding them during the first months of life when they’re too young to be vaccinated themselves. In this case, protecting the newborn outweighs the risk of waning immunity later in the season.
- Young children needing two doses: Children aged 6 months through 8 years who have never had a flu vaccine (or haven’t previously received at least two doses) need two shots spaced at least four weeks apart. Starting earlier ensures the second dose is finished before flu starts circulating widely.
- Anyone who can’t come back later: If your only opportunity to get vaccinated is in July or August, and you won’t be able to return in the fall, getting vaccinated early is better than skipping it entirely. Some protection that fades is still better than none.
Getting Your Flu and COVID Shots Together
If you’re also due for an updated COVID-19 vaccine, you can get both at the same visit. There’s no need to space them out, and combining the trips can simplify the process. The same September-to-October window works well for both, since COVID-19 also tends to surge during the winter months. RSV vaccines can also be given at the same time if your doctor recommends one.
What If It’s Already November or Later
Getting vaccinated after October is still worthwhile. Flu circulates well into spring, and even a late shot can protect you during the tail end of the season. The CDC recommends vaccination as long as flu viruses are still circulating. The “too early” concern disappears once you’re past October, and the only real mistake at that point is skipping it altogether.