Three doses of the vaccine against Covid can likely confer full protection, US infectious disease expert Dr Anthony Fauci has said, as the US is soon expected to officially roll out the booster dose.

There was good reason to believe that a third dose "will actually be durable, and if it is durable, then you're going to have very likely a three-dose regimen being the routine regimen", the CNN quoted him as saying at a briefing on Thursday.

Anthony Fauci
Dr Anthony FauciReuters

Fauci, the director of the National Institute of Allergy and Infectious Diseases, also cited two Israeli-based studies that showed a decrease in infections among people who got a third or booster shot.

The US Food and Drug Administration recently amended the emergency use authorisations for the Pfizer-BioNTech and Moderna Covid-19 vaccines to allow the administration of an additional dose to solid organ transplant recipients and other people who have an equivalent level of immunocompromise. But it hasn't been offered to all citizens.

"It's ultimately up to the US FDA to decide whether Americans should get three doses of the Covid-19 vaccine," Fauci said.

Meanwhile, both US drugmakers Moderna and Pfizer have applied to FDA for authorisation of a third dose either six months or eight months after getting the second dose.

The recommendation for the booster doses will likely lead to availability for a broad portion of the population, and doses could begin rolling out as early as the week of September 20, US Surgeon General Dr Vivek Murthy was quoted as saying.

"At some point down the line, we may have a way of telling who needs an extra shot, and who doesn't.

"Right now, we don't have that indicator, which is why we're recommending that not only people get vaccinated across the board -- regardless of whether they were infected in the past or not -- but also when it comes to getting these extra doses to sustain and extend your protection, that we do that broadly," he noted.

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