People vaccinated against COVID-19 may require booster shots within nine to 12 months of their initial vaccination, Reuters reported.

Evidence suggests that the coronavirus vaccines developed by Pfizer and Moderna offer at least six months of robust protection from COVID-19 infection. But even if this protection lasts for longer, several highly-transmissible viral variants are now circulating; that means people may need a regular booster shots to bolster immunity to the virus, Dr. David Kessler, chief science officer for President Joe Biden's COVID-19 response task force, said at a congressional committee meeting on Thursday (April 15).

"The current thinking is those who are more vulnerable will have to go first," in terms of getting their booster shots, Kessler said. 

Related: Quick guide: COVID-19 vaccines in use and how they work

From early on in the pandemic, experts predicted that officials may need to roll out multiple generations of COVID-19 vaccines and that regular booster shots may be necessary, since experts couldn't know then how long vaccine-generated immunity would last, Live Science previously reported. And there's always the potential for new virus variants to emerge and thwart the current vaccines, raising the risk of so-called breakthrough infections, meaning infections among fully vaccinated people.

Breakthrough infections are completely expected, as none of the COVID-19 vaccines are 100% protective against the coronavirus. The CDC has been monitoring for such infections since the vaccine rollout began, Dr. Rochelle Walensky, director of the U.S. Centers of Disease Control and Prevention, told the House subcommittee hearing, according to Reuters. 

Based on the number of vaccinated people who caught COVID-19 in clinical trials, and factoring in the attack rate of the virus, scientists would predict that about one in every 2,000 people who receive Pfizer or Moderna shots will still catch the virus, The Gothamist reported. (The attack rate is the likelihood of catching the virus after being exposed.) Since the Johnson & Johnson vaccine showed a lower level of protection in trials, the predicted infection rate would be closer to one in 300. 

When breakthrough infections do occur, however, the vaccines should frequently blunt the virus's blow and reduce the chance of a person developing severe symptoms, White House chief medical advisor Dr. Anthony Fauci said Monday (April 12) at a COVID-19 Task Force briefing, according to The Gothamist.

So far, about 5,800 breakthrough infections have occurred among 77 million fully vaccinated people in the U.S., Walensky said, as reported by Reuters. Of those infected individuals, 396 required hospitalization and 74 died.

Some vaccinated people may not mount a strong enough immune response following their shots, which can leave them vulnerable to breakthrough infections. It's also possible that the vaccines protect against certain variants of the virus better than others — hence the need for booster shots. Similar to the annual flu shot, the COVID-19 vaccines can be updated to better protect against new variants, and trials of such vaccines are already underway, Scientific American reported.

Originally published on Live Science.