The European Union’s top health regulator announced on Monday that it has approved booster shots for the COVID-19 vaccines from Moderna and Pfizer-BioNTech. The new approval applies for European citizens ages 18 and older from six months after their second dose.

In its statement, the European Medicines Agency (EMA) said that it concluded that an extra dose of the COVID-19 vaccines were effective after studies showed they improved on the ability to produce antibodies against the virus in organ transplant patients with weakened immune systems.

However, the EMA made a distinction between booster shots for those with compromised immune systems and those who do not suffer from those issues. 

For those with weakened immune systems, the agency based its recommendation on studies that showed an extra dose of these vaccines increased the 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 EMA said in a statement.

Among those with healthy immune systems, it evaluated data for Pfizer that showed a rise in antibody levels when a booster dose is given approximately six months after the second dose in people from 18 to 55 years old.

The EMA is the top health regulator in the 27-member bloc, but its recommendation has to be passed down to individual nations that decide how to apply them. Some members already began implementing booster shots before the EMA announcement, including France and Germany.

The EU’s recommendation comes amid discussions on whether or not citizens will need a booster shot to protect against the COVID-19 Delta variant that has been prevalent. Nations including Israel, the United Arab Emirates and the United Kingdom have rolled out booster programs. 

The FDA approved the Pfizer vaccine booster for Americans age 65 and older as well as those with underlying health conditions on Sept. 22. A decision on whether those who received a Moderna vaccine can receive a booster has not yet been announced.