It is very "likely" that people will need a third booster dose of the COVID-19 vaccines within 12 months of receiving their second dose, the CEO of Pfizer has said.

"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," Albert Bourla told a Facebook Live event moderated by a CNBC reporter on Thursday.

He added that "variants will play a key role".

It comes as the US Chief Medical Officer, David Kessler, told a congressional meeting on Thursday that the Biden administration was planning for such an eventuality.

"We are studying the durability of the existing vaccines to continue to mount an effective immunological response. Preliminary data show that neutralising antibodies persist for some time after the second dose of an mRNA vaccine with a relatively slow decline over time," he said.

"As with other vaccines, such as influenza vaccines, a subsequent dose may be important to provide continued protection against the wild-type strain but also may be critical to maintain protection against variants," he added.

Dr Anthony Fauci, the nation's top infectious disease official, also testified and said that so far, scientific evidence suggests that the COVID-19 vaccines distributed in the United States "continue to be effective against these variants".

US health authorities have so far approved three vaccines, respectively developed by Pfizer/BioNTech, Moderna, and Johnson & Johnson, the latter of which was temporarily suspended over rare blood clots concerns. So far, more than 37 per cent of the American population has received at least one dose.

But Fauci also highlighted that the National Institute of Allergy and Infectious Diseases launched clinical trials in late March involving "second-generation COVID-19 vaccines" that "could provide protection against emerging SARS-CoV-2 variants."

The European Union has also started preparing for the need to provide a third booster dose.

Commission President Ursula von der Leyen announced on Wednesday that Brussels had started negotiating with Pfizer to secure another 1.8 billion doses for delivery between 2021 and 2023.

"At a certain point in time, we might need booster jabs to reinforce and prolong immunity; and if escape variants occur, we will need to develop vaccines that are adapted to new variants; and we will need them early and in sufficient quantities," she said.