There's an ideal window to get your flu shot for the best protection: late October or the first week of November

• The CDC recommends getting a flu shot in September or October.
• But getting vaccinated in late October or the first week of November could maximize your protection.

Pharmacies start advertising flu shots as soon as they become available in August. The Centers for Disease Control and Prevention recommends getting a flu shot in September or October. But a narrower window may give the best chance of maximizing the vaccine's protection: late October or the first week in November.

An August study from CDC researchers found that on average, flu shots become 8-9% less effective at preventing hospitalization each month, based on data from four flu seasons between 2015 and 2019. Since flu vaccines are usually 40% to 60% effective in preventing illness (depending on the year), that means people can lose a significant amount of their protection after four to six months.

In the US, flu season can last as long as 8 months, from October to May.

David Cennimo, an infectious-disease expert at Rutgers Medical School, said the first week of November is probably a "sweet spot." People who get the flu shot much earlier run the risk of not having good protection during peak flu season - which is usually from December to March. People who get vaccinated later won't have enough time to develop protection once flu cases ramp up in late fall or early winter.

"It takes two weeks for the vaccine to be fully protective," Michelle Barron, medical director of infection prevention and control at UCHealth in Colorado, told Insider.

Barron said the first week of November is the latest she'd recommend. Late October is also ideal, she added.

"If you get it by the end of October, by the time Thanksgiving rolls around, you should have optimal protection when you're gathering with your family or getting on planes or other types of public transport," she said.

Peak flu season is tough to predict

The CDC recommends that people get vaccinated in September or October for two reasons, experts said: The agency wants to get as many people immunized as possible before flu season starts, so flu strains have less opportunity to spread. But it also recognizes that flu season is unpredictable, so cases could always take off earlier than average. The CDC anticipates "an early and possibly severe flu season" this year, for example, because fewer people got the flu last year.

Barron said she can recall years when flu cases took off in early November, prompting a "mad dash to get people vaccinated."

Cennimo said that although the most common peak is in February, "it's really almost impossible to predict when flu will peak in any given place in any given year."

This year's forecasts are especially difficult, since many areas still have COVID-19 restrictions that also reduce the spread of flu. What's more, countries in the Southern Hemisphere, which go through flu season first each year, recorded fewer cases than average.

"Australia is probably the one that we get our most robust data from, and Australia has still been basically in lockdown, so they haven't seen a significant amount of flu," Barron said.

Disease experts also aren't sure which areas of the US will get hit first.

"Influenza is a very local pandemic," Cennimo said. "Traditionally, California peaks weeks before [New York]. Flu tends to go west to east, though this year who really even knows?"

So public-health experts make the message simple: Get vaccinated before the end of October.

"The more confusing we make it, the more areas for error," Cennimo said.

A second shot or higher dose could offer more lasting protection

One group in particular may benefit from a delayed flu vaccine: adults age 65 and up. The CDC advises this group not to get vaccinated in July or August, since immunity wanes more quickly in older people. The August study found that flu shots are 10-11% less effective at preventing hospitalization each month for adults 65 and older.

But Cennimo said it's better for elderly people to get a high-dose flu vaccine than to play with the timing of the shot. A 2014 study found that the high-dose flu shot was 24% more effective in adults age 65 and older than the standard dose.

"I would go out of my way to find that [shot] in the store," Cennimo said.

In the future, he added, it's worth considering whether people would benefit from a two-dose flu shot, which might offer more long-term protection. He's also holding onto what he calls the "pipe dream" of a universal flu vaccine that could protect against all strains of influenza - perhaps for life.

"If we could get a universal vaccine, then we don't even have to do this every year," he said. "That would be amazing."