Pfizer's CEO said those who get the company's two-shot COVID vaccine will "likely" need a third shot within about a year's time. 

Albert Bourla, the head of the pharmaceutical company, said that in comments that were recorded April 1 but made public Thursday. 

“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 told CNBC’s Bertha Coombs during an event with CVS Health.

Researchers still don’t know how long protection against the virus lasts once someone has been fully vaccinated. Pfizer has said that it expects its vaccine to offer protection for at least six months.