A person holding a needle and a vial of flu vaccine.

After last winter’s severe flu season—and in the face of all those flu shot ads at pharmacies—you may be wondering if it's best to get vaccinated right now to safeguard yourself this winter.

After all, last year’s season set new records both for numbers of children who died from flu and for flu-related hospitalizations, according to the Centers for Disease Control and Prevention.

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But should you get the shot now, before the flu even arrives? Or might that undermine its effectiveness in January and February, when flu season is in full swing?

Because the immunity provided by the flu vaccine can wane over time, for most people, “October is probably the best time to get a flu shot,” says Andrew Pekosz, Ph.D., a professor of molecular microbiology at Johns Hopkins University Bloomberg School of Public Health. (That can help ensure that its effectiveness lasts through flu season, which can linger through March, and sometimes even longer.)

That’s also the recommendation of the CDC. According to the agency, it takes about two weeks after vaccination for your body to develop the antibodies necessary to start fighting flu. Getting vaccinated by the end of October means you’re likely to be fully protected by the time flu season begins to ramp up in the fall, and to stay protected throughout the season.

Still, it's not too soon to get vaccinated now if you think there’s a chance you might not get the shot next month. And if you miss getting vaccinated in October, it’s not too late to do so throughout flu season.

A few groups, however, may want to time their flu vaccination a little more carefully. Here, what to consider for young children and older adults, plus an early look at what the upcoming flu season might be like.

Who Needs the Flu Shot, and When

When it comes to kids, the American Academy of Pediatrics recommends that children receive the flu shot as soon as it's available. And experts say that getting vaccinated before October is an especially good idea for children 6 months to 8 years old who’ve never had the flu shot before, or who have been vaccinated only once in their lives. (Kids should receive their first flu vaccine at 6 months of age.) 

That’s because, unlike the rest of us, those youngsters require two doses of the flu vaccine given 28 days apart, says William Schaffner, M.D., a professor of medicine at Vanderbilt University School of Medicine and a consultant to the CDC’s Advisory Committee on Immunization Practices. So if they get their first shot in early to mid-September and their second in early to mid-October, they’ll probably be protected by the time the flu arrives.

Two groups might benefit from waiting a bit before getting the shot: people 60 and older and those who have a compromised immune system due to conditions such as autoimmune disease or HIV, or who are undergoing chemotherapy treatment.

Research suggests that for at least some people in those groups, the immune system’s virus-fighting ability begins to drop after about four months—sooner than it may for others.

This evidence is not definitive, however, and even if the vaccine’s strength does decrease, it still seems to provide some protection after the four-month mark, Schaffner says.

Still, people who are older or have underlying conditions like those mentioned above might want to wait until late September or early October to be vaccinated—and to be sure that their protection will cover as much of flu season as possible.

What Will This Flu Season Look Like?

While predicting with certainty the severity of an upcoming flu season is impossible, according the CDC, influenza experts say there are a few signs that may provide some information.

One potential clue: Countries in the Southern Hemisphere, like Australia, where spring is just beginning, are reporting relatively mild flu seasons, Pekosz says.

The current formulation of the flu shot being used there (which is the same as the U.S. vaccine) appears to be well-matched with the viruses circulating there.

That could mean we’re in store for a mild season in the Northern Hemisphere, too—as long as those strains are the same ones that crop up here.

Another good sign is that so far, the strains of flu virus currently circulating in southern countries appear similar to the strains that predominated during last winter’s season here (notably, subtypes of influenza B and the H1N1 and H3N2 strains of influenza A), says David Topham, Ph.D., director of the New York Influenza Center of Excellence and professor of microbiology and immunology at the University of Rochester Medical Center.

So people who caught the flu last year may have some residual immunity to help protect them this year, he says.

But we don't yet know for sure what strains will end up circulating here this season. And a mild flu season in Australia shouldn’t lull anyone into complacency, Pekosz says.

The virus strains circulating in the Southern Hemisphere right now could change or mutate—which flu strains have been known to do. If that happens, this year’s flu vaccine and the immunity people have left over from last year may not be as protective as hoped.

So, it’s critical for everyone 6 months and older to be vaccinated against the flu. That’s especially true for children, pregnant women, older adults, and people with underlying health conditions, all of whom are at a greater risk for severe illness if they do get the flu.