With the Food and Drug Administration’s clearance, Pfizer and Moderna are set to begin shipping millions of doses. A third U.S. manufacturer, Novavax, expects its modified vaccine version to be available a little later.

“We strongly encourage those who are eligible to consider receiving an updated COVID-19 vaccine to provide better protection against currently circulating variants,” said FDA vaccine chief Dr. Peter Marks.

The agency’s decision came a bit earlier than last year’s rollout of updated COVID-19 vaccines, as a summer wave of the virus continues in most of the country. The Centers for Disease Control and Prevention already has recommended this fall’s shot for everyone age 6 months and older. Vaccinations could be available within days.

While most Americans have some degree of immunity from prior infections or vaccinations or both, that protection wanes. Last fall’s shots targeted a different part of the coronavirus family tree, a strain that’s no longer circulating—and CDC data shows only about 22.5% of adults and 14% of children received it.

Skipping the new shot is “a hazardous way to go,” because even if your last infection was mild, your next might be worse or leave you with long COVID symptoms, said Dr. Robert Hopkins Jr. of the National Foundation for Infectious Diseases.

This fall’s vaccine recipe is tailored to a newer branch of omicron descendants. The Pfizer and Moderna shots target a subtype called KP.2 that was common earlier this year. While additional offshoots, particularly KP.3.1.1, now are spreading, they’re closely enough related that the vaccines promise cross-protection. A Pfizer spokesman said the company submitted data to FDA showing its updated vaccine “generates a substantially improved response” against multiple virus subtypes compared to last fall’s vaccine.

The big question: How soon to get vaccinated? This summer’s wave of COVID-19 isn’t over but the inevitable winter surges tend to be worse. And while COVID-19 vaccines do a good job preventing severe disease, hospitalization and death, protection against mild infection lasts only a few months.

People who are at high risk from the virus shouldn’t wait but instead schedule vaccinations once shots are available in their area, Hopkins advised.

That includes older adults, people with weak immune systems or other serious medical problems, nursing home residents and pregnant women.

Healthy younger adults and children “can get vaccinated anytime. I don’t think there’s a real reason to wait,” Hopkins said – although it’s OK to seek the shots in the fall, when plenty of doses will have arrived at pharmacies and doctor’s offices.

The exception: The CDC says anyone who recently had COVID-19 can wait three months after they recover before getting vaccinated, until immunity from that infection begins to wane.

Hopkins, who sees patients at the University of Arkansas for Medical Sciences, calls it vital for more youngsters to get vaccinated this year—especially with schools starting as coronavirus levels are high around the country.

“COVID does not kill many children, thank goodness, but it kills far more children than influenza does,” Hopkins said, adding that teachers, too, should quickly get up to date with the vaccine.

Health authorities say it’s fine to get a COVID-19 and flu vaccination at the same time, a convenience so people don’t have to make two trips. But while many drugstores already are advertising flu shots, the prime time for that vaccination tends to be late September through October, just before flu typically starts its cold weather climb.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.

More on COVID-19 and vaccines:

Subscribe to Well Adjusted, our newsletter full of simple strategies to work smarter and live better, from the Fortune Well team. Sign up for free today.