The European Medicines Agency, the European Union’s main drug regulator, said on Monday that a booster shot of the Pfizer-BioNTech coronavirus vaccine could be given to healthy adults at least six months after the second dose.

The agency said data showed that antibody levels increased in people age 18 to 55 with normal immune systems who had received a third dose of the vaccine. It is still assessing booster shots of the Moderna vaccine.

The agency also said that people with “severely weakened” immune systems could receive an extra dose of the Pfizer-BioNTech or Moderna vaccines as early as 28 days after a second dose. It is expected that an additional shot “would increase protection in at least some of the patients,” the agency said.

The recommendation is based on studies showing that an extra dose of those vaccines could increase the ability to produce antibodies in organ transplant recipients.

In the European Union, vaccination campaigns are a prerogative of national governments, and each of the 27 member nations can decide whether to give booster shots to all of their adult residents. Some E.U. nations, such as France, Germany and Belgium, started giving extra doses to older people and those with weakened immune systems last month, while the Czech Republic and Hungary opened this possibility to all adults.

Although the European Union has one of the highest vaccination rates in the world, with over 73 percent of adults fully inoculated, there is no coronavirus vaccine authorized yet for children. The European Center for Disease Prevention and Control warned last week that the average level of vaccination across the bloc is not sufficient to halt the virus from spreading if governments relax Covid-19 restrictions.

The agency said it was carefully monitoring “very rare” side effects of a booster shot, such as inflammatory heart conditions, but that for the moment the risk was not known.

The decisions of wealthier nations to administer booster shots while the rest of the world remains largely unvaccinated have raised alarm among health experts. Dr. Tedros Adhanom Ghebreyesus, the director general of the World Health Organization, has called for a moratorium on coronavirus vaccine booster shots for people who are not immunocompromised until at least the end of the year.