Efforts to give a Covid-19 booster shot to certain adults in the United States have officially begun, following a week of intense deliberation from government advisory committees.

Early Friday, Centers for Disease Control and Prevention Director Dr. Rochelle Walensky signed off on a booster dose for older adults, as well as people at high risk of exposure or severe disease.

While a third shot can officially start going into arms, there is a caveat: Only people who have already been fully immunized with Pfizer-BioNTech’s two-dose Covid vaccine are eligible for a booster shot, which must also be Pfizer’s, based on the decisions made this week.

That leaves out millions of Americans who were initially vaccinated with Covid vaccines made by either Moderna or Johnson & Johnson — a decision that frustrated a number of advisers at a meeting of the CDC’s advisory committee, which met Wednesday and Thursday to discuss the booster recommendations.