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

The nation’s last flu season was less severe than the 2017 to 2018 season, which sickened an estimated 48.8 million people. But it was still plenty dangerous.

Early data suggests that it caused between 37 million and 43 million illnesses, and an estimated 36,400 to 61,200 deaths in the U.S. And it was the longest flu season in a decade, according to the Centers for Disease Control and Prevention.

No one is sure what the upcoming flu season will bring, but there is certainty that the flu vaccine continues to be the best way to protect yourself from getting sick. So, should you get that flu shot now? 

According to William Schaffner, M.D., a professor of medicine at the Vanderbilt University School of Medicine, getting it later in September or October may be the best choice for most people.

Here’s what to know and how to decide.

What’s the Best Timing for You?

The CDC has long advised that all those who are over 6 months old, with very few exceptions, should get their flu shots by the end of October. That way, the vaccine has a chance to become fully effective—which can take two weeks—before flu season hits. 

But is getting the vaccine (which is often available in July or August) somewhat earlier a potential problem?

Perhaps. In its recommendations for this year, the CDC noted that very early vaccination—before September—could lead to less-than-optimal protection before flu season ends. 

“Balancing these considerations, July and August may be too early, particularly for older adults,” says Lisa Grohskopf, M.D., a flu expert at the CDC. Flu vaccines can produce less robust immune responses in older people compared with younger adults.

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That’s because some research suggests that the shot’s effectiveness might wane after a few months, possibly leaving people who got it early less shielded at the tail end of the season. 

“There’s some legitimate concern that there could be some declining protection over time,” says Edward Belongia, M.D., epidemiologist and vaccine researcher at the Marshfield Clinic Research Institute in Wisconsin. 

In a study published in 2017 in Clinical Infectious Diseases, for instance, CDC researchers looked at several recent seasons and found that the longer it had been since someone received the flu shot, the more likely he or she was to get sick. (The drop-off in protection was modest—a decrease of about 7 percent per month for H3N2, a strain known to make large numbers of people quite ill.)

Earlier research published in 2015 in the journal Vaccine, which looked back at the 2007 to 2008 season, found that protection against H3N2 seemed to drop slightly over the course of the season in young children and older adults in particular.

But other studies haven’t found that immunity provided by the flu vaccine diminishes substantially within one season. And other factors may play a role, says Belongia, lead author of the Vaccine study. For instance, flu strains can mutate, so a vaccine that was effective against it earlier in the season may become less so once a strain changes.

Even though the research on waning effectiveness isn’t conclusive, it’s probably wise to do as Schaffner recommends, and opt for the vaccine later in September or at some point in October. This is late enough to confer protection throughout the flu season but early enough to safeguard you in case the virus starts circulating sooner than usual. 

There are special considerations for some kids. If you have children between the ages of 6 months and 8 years who’ve never had a flu shot, or who’ve been vaccinated only once before, you may want to get them in for the shot in September.  

That’s because, unlike the rest of us, those youngsters require two doses of the flu vaccine given at least 28 days apart, Schaffner says. So if they get their first shot in late August or September and their second in October, they’re likely to be protected by the time the flu arrives.

The most important thing, though, is to get the shot when you can, even if it’s on the early side. As Schaffner says, “A vaccine deferred is often a vaccine that’s not received.” 

What Will This Flu Season Be Like, Anway?

Making predictions about future flu seasons is notoriously difficult. 

Sometimes, flu activity in Australia, which is near the end of its flu season right now, can foreshadow what may happen in this country, Schaffner says. 

Australia has had an unusually high number of flu cases so far this year. (Its season got off to an early start.) Still, based on the number of intensive care unit admissions and deaths from flu there so far this year, Australia’s season hasn’t been especially severe, according to public health officials [PDF] there. 

But the flu is unpredictable, and there’s no way to know whether the U.S. flu season will be similar to Australia’s. 

This underscores the importance of protecting yourself against the virus for whatever may come throughout the 2019 to 2020 flu season. And that means getting a flu shot. 

That’s especially important 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.

Julia Calderone contributed to the reporting of this story.