Europe's drug regulator advises Covid-19 booster shots for the immunocompromised

(CNN) The European Medicines Agency (EMA) has concluded that a booster shot of the Pfizer/BioNTech and Moderna coronavirus vaccines "may be given to people with severely weakened immune systems, at least 28 days after their second dose," according to a statement Monday.

The regulator said it had come to the decision to recommend a third shot after studies showed an extra dose of the two vaccines "increased the ability to produce antibodies against the virus that causes COVID-19 in organ transplant patients with weakened immune systems."

The EMA statement added that there was "no direct evidence" that being able to produce antibodies protected against coronavirus, but "it is expected that the extra dose would increase protection at least in some patients."

The agency said it would continue to watch for data on the effectiveness of protection.

It also drew a distinction between giving the extra dose to people with compromised immune systems and those without. For people with normal immune systems, the EMA concluded that booster doses of BioNTech/Pfizer "may be considered at least 6 months after the second dose for people aged 18 years and older."