How often do you need a COVID booster shot? Yearly, new data suggests.

Healthy people should get annual COVID-19 boosters to prevent widespread outbreaks, suggests a new study from Yale University and the University of North Carolina at Charlotte.

Yearly shots provide just enough frequency to prevent huge outbreaks while not putting an undue burden on people.

"There seems to be an inflection point" at about a year, said senior author Jeffrey Townsend, a biostatistician and evolutionary biologist at the Yale School of Public Health. “Delaying boosting beyond that point rapidly increases the risk of an infection."

Though federal officials have suggested annual shots, this study is the first to examine the long-term outcomes from a booster schedule and the first to show that boosters will be especially effective at intervals of no more than a year.

The study, published this month, focused on people with healthy immune systems. Townsend and his colleagues are starting work on a similar study looking at the optimal vaccine interval for people with weakened immunity from cancer treatment and other health problems.

What the study found: Annual shots prevent 75% of infections

The researchers modeled antibody levels against the virus that causes COVID-19 if someone was boosted with a Pfizer-BioNTech or Moderna vaccine every six months, one year, 18 months, two years or three years over a six-year time span.

Boosting twice a year reduced infections by more than 93%, the team found, but Townsend said it's not realistic to ask people to get vaccinated that often.

Annual shots prevented 75% of infections, while waiting six more months reduced the rate of protection to just 55%. Shots every three years prevented just 24% of infections, the study found.

"Delaying the administration of updated boosters has bleak repercussions," it concluded.

Boosters won't prevent all infections

For healthy people, "annual boosting really makes a difference," said Alex Dornburg, an expert in bioinformatics at the University of North Carolina Charlotte and a co-author of the study.

Roughly 3 in 10 people will get infected with COVID-19 even with an annual shot, Dornburg said. But 9 in 10 will be infected without such updates.

Not getting boosted triples the risk for infection over six years, co-author Hayley Hassler of Yale said.

The Centers for Disease Control and Prevention and most infectious disease experts encourage people of all ages to get boosters.

The booster has to keep evolving, too

The virus continues to evolve, "so we need to continually evolve the booster and keep it updated to the most prevalent variant," Hassler said.

Over time, people will lose the ability to respond effectively to the virus without boosters or reinfections, Townsend said.

The latest booster is directed at both the original coronavirus and the BA.4 and BA.5. Although these are not the most common variants anymore, the researchers said, the new booster is close enough to provide a benefit.

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Getting vaccinated vs. being infected

The COVID-19 vaccines have side effects; they can make people feel miserable for a day or two. But an infection can be much worse, Dornburg said.

In his own case, he felt terrible for two days after his first round of shots. When he caught COVID-19, though, his lungs hurt for weeks and he was winded climbing a single flight of stairs.

"It was awful," he said. "That calibrated me a little bit differently."

Contracting COVID-19 does provide protection against future infections, but like the vaccine, the protection fades over time, at roughly the same pace, Townsend said.

And it's better not to get infected at all.

"You can't end up in the hospital for an infection you didn't get," Townsend said.

In another study published Wednesday, researchers from the World Health Organization examined 26 earlier papers and concluded that people who are vaccinated or infected remain better protected against severe disease a year later compared with those who are unvaccinated and never infected.

A year after someone is both vaccinated and infected, they have a 95% reduced risk of getting severe COVID-19 or needing to be hospitalized, the study showed. Someone infected a year ago but not vaccinated had a 75% reduced risk.

Getting infected and vaccined reduces the chance of reinfection a year later by 42%. Someone who was infected but not vaccinated had a 25% lower chance of reinfection.

When will we have better vaccines?

Unfortunately, said Dr. Eric Topol, founder and director of the Scripps Research Translational Institute, the federal government has lost its momentum on vaccine development.

"We need durable vaccines that would last multiple years against all variants," said Topol, who was not involved in the new research. "We're not getting the traction we need on these science projects. There's not the will, the resources, the priority."

That means if a worse variant arises at some point, the U.S. "will have to be reactive instead of having all this ahead of time – and that would be really sad because we know the path to getting ahead of the virus and we're not doing it," he said.

Topol believes nasal vaccines are needed to prevent more infections. Even if a nasal vaccine has to be given more often than shots, people won't mind as much.

"I'm excited about it. I haven't seen anything to negate it," he said. "I'd be happy to take a spray every few months to prevent infection."

Contact Karen Weintraub at kweintraub@usatoday.com.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: When to get a COVID booster? Data says annually to prevent outbreaks