AMSTERDAM (AP) — The European Union’s drug regulator gave its backing Monday to booster shots of the Pfizer-BioNTech COVID-19 vaccine for people 18 and older.

The European Medicines Agency said the booster doses “may be considered at least 6 months after the second dose for people aged 18 years and older.”

The agency also said it supports giving a third dose of either the Pfizer-BioNTech or the Moderna vaccine to people with severely weakened immune systems at least 28 days after their second shot.

The recommendations go to health authorities in all 27 EU member states. Some countries already have begun administering booster shots.