Pfizer CEO Albert Bourla has suggested that it’s “likely” people will be needing a third dose of a COVID-19 vaccine within 12 months of being fully vaccinated.

The Pfizer-BioNTech vaccine, like most other COVID-19 vaccines with the exception of Johnson & Johnson, requires two shots. Bourla has suggested that people might have to get vaccinated against the coronavirus every year, although it’s likely they’d only need to get one more shot rather than two.

“We need to see what would be the sequence, and for how often we need to do that, that remains to be seen,” said Bourla, per CNBC. “A likely scenario is that there will be likely a need for a third dose, somewhere between six and 12 months and then from there, there will be an annual revaccination, but all of that needs to be confirmed. And again, the variants will play a key role.”

He added that it is “extremely important to suppress the pool of people that can be susceptible to the virus.” His comments come not long after Johnson & Johnson CEO Alex Gorksy said in February that people might have to get vaccinated annually, similar to seasonal flu shots. Researchers have suggested it’s unclear how long protection against the virus lasts once someone has been fully vaccinated. 

David Kessler, the Biden administration’s COVID-19 response chief science officer, said on Thursday that Americans can expect to receive booster shots to help protect them against any possible variants of the novel coronavirus. The litany of new variants threaten to “challenge” the effectiveness of the shots, he added. “We don’t know everything at this moment. … We are studying the durability of the antibody response.

Pfizer and BioNTech previously announced tests for the third dose of their vaccine were underway to help further understand how the immune response battles new variants. On Wednesday, Moderna revealed that the company is hoping to have a booster shot for its vaccine available by fall, CNBC notes.