WASHINGTON — The nation’s top doctor says it’s “certainly possible” that Americans eventually will be advised to get a booster shot of the coronavirus vaccine.

Surgeon General Vivek Murthy told CNN that no decision had been made yet after a meeting Monday with Pfizer to discuss its request for approval of a third shot of its coronavirus vaccine. He added as officials make their assessments, “what we’re really looking for is clear evidence that immunity is waning,” such as breakthrough infections in the vaccinated population. He says that may trigger a recommendation for booster shots.

Murthy also says it’s “certainly very possible” a third shot will be recommended for immunocompromised or vulnerable Americans, such as those on chemotherapy.

“We are looking closely at that population,” he says, adding when the data suggests a booster dose is needed, it will be recommended.