Five days after an FDA advisory committee recommended a booster dose of Pfizer Inc.-Biontech SE’s COVID-19 vaccine, Comirnaty, for people 65 and older or those who are at high risk of a serious infection, the CDC’s Advisory Committee on Immunization Practices (ACIP) struggled Sept. 22 with what a U.S. booster program would look like if only one booster is available when three different COVID-19 vaccines are being used in the country.