The U.S. Food and Drug administration on Tuesday authorized a second booster dose of the Pfizer-BioNTech and Moderna COVID-19 vaccines for use in adults ages 50 and older, NBC News reports.

Individuals are eligible for a second booster dose at least four months after receiving their first booster, the FDA said, per NBC News. It does not matter which brand of vaccine a person initially received.

Centers for Disease Control and Prevention Director Rochelle Walensky is expected to sign off on the decision shortly. The FDA had already authorized a fourth shot for immunocompromised Americans, per NBC News.

The CDC will likely say people older than 50 can get a second booster, rather than explicitly recommending they do so, The Washington Post reports, “a reflection of the ongoing debate about the benefits of additional doses and uncertainties about the future of the pandemic.” The federal agency will also likely “highlight vulnerable populations” within the designated age group.

Even with the FDA and (eventually) CDC’s decision, health officials “likely face an uphill battle in another vaccine campaign,” notes The Wall Street Journal. Approximately two-thirds of adults over age 65 have received a booster, and less than half of the adult population has.

The extra shots will be available through “physician offices, retail pharmacies, nursing homes and other vaccination sites,” per the Journal. A fourth shot is expected to be authorized for the broader population this fall.