The Biden administration is rolling out coronavirus vaccine booster shots for certain groups after the Centers for Disease Control and Prevention endorsed a third dose of the Pfizer-BioNTech vaccine for Americans over 65 and adults at heightened risk of severe illness.

The administration had announced plans to make booster shots available to all Americans, citing concerns about people’s immunity from shots waning over time. The decision comes amid a surge in infections from the delta variant across the country, all while vaccinations rates have slowed.

Qualifying Pfizer-BioNTech vaccine recipients can get a booster six months after the date of their second dose. Officials said they will address recommendations for the Moderna and Johnson & Johnson vaccines after reviewing relevant data.

Here’s what you need to know about booster shots and their rollout.