The European Union's drugs regulator said on Monday people with weakened immune systems should get a third dose of a Covid-19 vaccine from Pfizer-BioNTech or Moderna, but left it to member states to decide if the wider population should get a booster.
The long-awaited guidance comes after several EU member states pre-empted the European Medicines Agency's (EMA) opinion and launched their own booster campaigns, although they vary widely over who is eligible.
The EU joins the United States, Britain and Israel where regulators have approved the use of Pfizer boosters, although there is no consensus among scientists about how broadly they should be deployed. Israel is the outlier, using them across the whole population.
Governments are under pressure to revive their ailing economies, fight the more infectious Delta coronavirus variant, and avoid further lockdowns in the winter.
The EMA's ruling comes after the EU's infectious diseases centre warned last week the region's coverage of vaccines was still too low and there was a risk of a significant surge in cases, hospitalisations and deaths over the next six weeks.
The EMA said people with a severely weakened immune system should be given a third dose of the vaccines from Pfizer-BioNTech and Moderna - both based on mRNA technology - at least 28 days after their second one.
It also said a booster shot of the Pfizer/BioNTech vaccine could be considered for adults with normal immune systems around six months after the second dose, but added that was up to individual countries.
"At national level, public health bodies may issue official recommendations on the use of booster doses, taking into account emerging effectiveness data and the limited safety data," it said.
The World Health Organization has criticised rich nations for hoarding COVID-19 vaccines for booster campaigns for larger population groups, while poorer countries are struggling to rollout even first doses.
Allowing EU countries to decide broader use of a booster is consistent with the EMA's earlier decisions in the pandemic - for instance it largely left it up to member states to decide whether to restrict vaccines in the face of potential side effects.
