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’s human medicines committee issued the recommendation after studying data for the Pfizer vaccine that showed a rise in antibody levels following boosters given around 6 months after the second dose in people from 18 to 55 years old.

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 agency said its decision came after studies showed that an extra dose of the vaccines increased people’s ability to produce antibodies against the virus that causes COVID-19 in organ transplant patients with weakened immune systems.

“Although there is no direct evidence that the ability to produce antibodies in these patients protected against COVID-19, it is expected that the extra dose would increase protection at least in some patients,” the agency said in a statement.

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

Numerous studies have shown that the Pfizer-BioNTech and Moderna vaccines remain strongly protective months after people receive their second dose, dramatically cutting the risk of hospitalization and death.

The World Health Organization’s chief had urged wealthy nations not to use booster doses this year, saying there is no scientific data that proves the shots are necessary. He said COVID-19 vaccines would be put to better use in developing countries, where many people still have not received their first vaccine shots.

The United States government launched a campaign last month to offer boosters of Pfizer’s COVID-19 vaccine to millions of Americans even as federal health officials stressed the real problem remains getting first shots to the unvaccinated.

The coronavirus pandemic has killed at least 4.8 million people around the world.