Top infectious disease official Anthony Fauci said on Sunday that Americans who are immune compromised may end up needing COVID-19 vaccine booster shots as the United States deals with increasing cases from the Delta variant of the coronavirus.

“Those who are transplant patients, cancer chemotherapy, auto-immune diseases, that are on immunosuppressant regimens, those are the kind of individuals that if there’s going to be a third booster, which might likely happen, would be among first the vulnerable,” Fauci said during a CNN interview.

He added health officials are also considering whether to revise mask guidance for vaccinated Americans saying it was “under active consideration.”