Within the next 48 hours, the US Food and Drug Administration is expected to authorize a Coronavirus booster shot for people who are immunocompromised.

According to CNN, a source with the FDA said they are closely watching studies in which a third dose is given to immunocompromised people. This booster shot would be a third dose added to the two-shot Pfizer and Moderna vaccines.

All three vaccines currently in use have been approved for emergency use, but the FDA has to authorize using them in a new way outside the existing approval. About 9 million Americans are immunocompromised, according to an estimate by the CDC.

Vaccine advisors with the CDC, according to an agenda released Monday, are expected to meet Friday to discuss the new possible booster shot.