WASHINGTON: The head of Pfizer said in an interview aired Thursday that people will "likely" need a third dose of his company's Covid-19 vaccine within a year of being fully vaccinated.

CEO Albert Bourla also said annual vaccinations against the coronavirus may well be required.

"We need to see what would be the sequence, and for how often we need to do that, that remains to be seen," Bourla told CNBC in an interview recorded on April 1.

"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," he said, adding that variants will play a "key role."

"It is extremely important to suppress the pool of people that can be susceptible to the virus," he said.

Researchers currently don't know how long vaccines provide protection against the coronavirus.

Pfizer published a study earlier this month that said its jab is more than 91 percent effective at protecting against the coronavirus, and more than 95 percent effective against severe cases of Covid-19 up to six months after the second dose.

But researchers say more data is needed to determine whether protection lasts after six months.

David Kessler, the head of US President Joe Biden's Covid-19 response team, warned a congressional committee on Thursday that Americans should expect to receive booster shots to defend against coronavirus variants.

"We don't know everything at this moment," he told the House Coronavirus Crisis Subcommittee. "We are studying the durability of the antibody response.

"It seems strong but there is some waning of that and no doubt the variants challenge," he said.

"I think for planning purposes, planning purposes only, I think we should expect that we may have to boost."

The Pfizer vaccine, developed in partnership with German firm BioNTech, currently plays a leading role in American and European vaccination campaigns.

The pharmaceutical giant announced in February that it was testing a third dose of its vaccine to better combat the emerging variants.