After virtually disappearing for two years in the U.S. as the COVID-19 pandemic shut down society, there are hints the flu could reemerge this fall, potentially causing an unusually early and possibly severe flu season.

As a result, many experts are urging people to get their flu shots right away to make sure they're protected. But is that the best timing?

It depends on which expert you talk to and, maybe, on your age and particular situation.

"It's time to get your flu shot right now," advises Dr. William Schaffner, a professor of infectious disease at Vanderbilt University.

"People should get them now," agrees Shaun Truelove, an assistant scientist at the Johns Hopkins Bloomberg School of Public Health who's helping lead a new effort to project this year's flu season for the Centers for Disease Control and Prevention.

The usual flu season starts in November in the U.S. and peaks in January or February. "In normal years, it makes sense to hold off on the flu shot until late fall, as protection really doesn't last more than a few months and late fall/winter is when the flu wave usually hits here," says Deepta Bhattacharya, an immunologist at the University of Arizona College of Medicine. "So in a normal year, I would probably try to wait until mid-October and get the flu shot then," he says. But this year, he says, "flu cases are already starting to go up, so it makes sense to get it sooner — i.e., now."

Why flu could come back strong this year

The reason experts are particularly concerned about the flu this year is that many people, especially very young children, may have little or no immunity against the respiratory infection because the masking, social distancing and other behaviors aimed at protecting against COVID have blunted flu's spread, too. Also, the CDC notes, young children would do well to get a flu shot soon because they require two shots one month apart, and it takes time to build up immunity.

Because the protection that flu vaccination provides to the elderly tends to wane faster than for other adults, many experts recommend against that group getting vaccinated too early — but one now is fine.

"For the elderly, I would not recommend vaccination until September at the earliest, since immunity can wane. I tend to get my vaccination in October," says Dr. Walter Orenstein, a vaccine specialist at Emory University who previously worked at the CDC.

Warning signs from Australia

There's another reason for many other adults to get a flu shot sooner than later: The flu season hit early and hard in some countries in the Southern Hemisphere (such as Australia) this past winter. And what happens during the winter south of the equator often foreshadows that will happen in the Northern Hemisphere.

"So getting the vaccine earlier is definitely a good idea," Truelove says.

Still, some experts say they're planning to wait several weeks yet to get their own shot.

"I'll get mine sometime in November," says John Moore, an immunologist at Weil Cornell Medicine. "Protection by flu vaccines is usually weak and short-lived," he notes, "so getting vaccinated too early provides too little protection when the virus is actually circulating. And that's not now. We are not in 'flu season' yet."

Others say adults might reasonably wait until it seems like cases in their region are increasing.

"I'd say it's best to keep eyes on what flu activity is like in your area," says Jeremy Kamil, an immunologist at Louisiana State University. "Ideally, we'd want to match our boosting to afford us optimum levels of immunity when the virus is actually on the rise."

Just don't wait too long. Because the flu often starts to hit hard right when people are traveling and gathering for Thanksgiving, the National Foundation for Infectious Diseases encourages people to "make sure they're vaccinated by Halloween," says Patsy Stinchfield, the foundation's president. "It takes about two or three weeks for antibodies to rise high enough to be protective."

In the end, the precise timing of when you get a flu shot over the next month or so doesn't matter as much as that you get one, flu experts agree, especially this year. Truelove's group estimates that, in the worst case scenario, the flu could hospitalize as many as 560,000 people in the U.S. this year — and kill as many as 63,000.

"A vaccine deferred is often a vaccine not received," Schaffner says. "You have to have the discipline to be sure that you do get vaccinated."