White House chief medical adviser Anthony Fauci says three shots are likely to be required to ensure that coronavirus vaccines remain effective for a long time.
Fauci spoke to reporters on Thursday and referred to Israel, which has already started offering a third dose.
He noted that data from that country shows a significant reduction in the risk of infection and severe disease. He said the data very strongly supports the rationale for booster shots.
Fauci said a third shot "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 US government says coronavirus vaccines may become less effective with the passage of time or due to the spread of the Delta variant.
An expert panel of US regulatory authorities is discussing a plan to give booster shots after a certain period.
Some countries have already started rolling out third doses. The World Health Organization has called for a delay in administering boosters, saying this will affect global vaccine supplies.
EU authorities also say the efficacy of a third dose is not yet clear and there is no need for haste in giving booster shots to ordinary people.