When should I get my flu shot?

A man receiving a flu shot from a doctor
A man receiving a flu shot from a doctor (Image credit: Jose Luis Pelaez Inc via Getty Images)

Fall has arrived, and with it the annual call to get your flu shot. But when's the best time to get the jab?

Ideally, it’s better to get vaccinated before flu starts to circulate in your community; but exactly when flu season will start is unpredictable, according to the U.S. Centers for Disease Control and Prevention (CDC). For this reason, the CDC recommends that people who are eligible for a flu shot — generally everyone ages 6 months and older —  get it in September or October, preferably by the time Halloween rolls around, as flu typically isn't widespread yet in these early fall months. In the U.S., flu activity often begins to tick up in October and typically peaks between December and February, according to the CDC.

After you get a shot, you aren't protected right away — it takes about two weeks for the body to build up immunity against the flu, which is another reason not to wait until the flu hits your area to get vaccinated.

If you aren't able to get your shot by the end of October, vaccination later in the season is still recommended. Flu activity in the U.S. can sometimes linger until May, according to the CDC.

Related: Which flu shot should I get?

However, there are signs that flu season could start early this year in the U.S., based on what was seen in the Southern Hemisphere, which experiences its flu season during its winter (which is summer in the Northern Hemisphere). For instance, Australia’s flu season normally peaks between July and September, but this year, the country saw flu activity increase in April and peak in May and June, according to UCHealth, a nonprofit healthcare system in Colorado. 

And flu activity is already increasing in some U.S. states — as of late September, Texas and Georgia are reporting moderate levels of flu activity, and the District of Columbia is reporting very high flu activity, according to the CDC.

The flu shot’s level of protection against the flu does wane with time — a 2017 study published in the journal Clinical Infectious Diseases found that the flu vaccine's effectiveness decreased by about 6% to 11% per month, but the shot still offered some level of protection for five to six months after vaccination.

Because of the waning protection, the CDC recommends against getting vaccinated in July or August for most adults. But there are some exceptions, including pregnant women in the third trimester, who may consider vaccination in July or August in order to pass protective antibodies onto their babies after birth. In addition, some children need two doses of the flu vaccine given four weeks apart, and so these children should receive their first flu vaccination as soon as the vaccine becomes available (including in July or August) in order to get the second shot by the end of October, according to the CDC. (Children who need two doses include those ages 6 months to 8 years old who are getting the flu shot for the first time, and those who only ever received one flu shot, according to the CDC.)

In addition to the flu vaccine, there's another vaccine being offered to the general public this fall — the COVID-19 bivalent booster shot, which is currently recommended for people ages 12 and older. If you want to save time and get both the flu shot and COVID-19 booster shot together, it's safe to do so, according to the CDC. People may be slightly more likely to experience systemic side effects, such as fatigue and muscle aches, if they receive both shots at the same time, But these side effects tend to be mild and resolve quickly, Live Science previously reported.

Originally published on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.