A bottle of Moderna COVID-19 vaccine on a table before being utilised in Topeka, USA. AP
Spikevax is the second booster to be given the green light after Pfizer/Biontech's Comirnaty jab earlier this month.
"Data showed that a third dose of Spikevax given six to eight months after the second dose led to a rise in antibody levels in adults whose antibody levels were waning," the European Medicines Agency said.
Public health bodies within the 27-member bloc "may issue official recommendations on the use of booster doses, taking into account the local epidemiological situation", the Amsterdam-based EMA said.
It added that current data showed a pattern of side effects similar to that which followed a second dose of Spikevax.
"The risk of inflammatory heart conditions and other very rare side effects after a booster is being carefully monitored."
Earlier this month, the EU medicines watchdog approved booster shots of Comirnaty for over-18s as well as extra doses of Comirnaty and Spikevax for people with severely weakened immune systems.