The European Medicines Agency said Wednesday that Johnson & Johnson's Covid vaccine can be used as a booster shot two months after the first dose was administered, or after receiving other mRNA shots.

"EMA's human medicines committee (CHMP) has concluded that a booster dose of Covid-19 vaccine Janssen may be considered at least two months after the first dose in people aged 18 years and above," the watchdog said in a statement, sing the vaccine's commercial name.

This is the third vaccine after Pfizer/BioNTech and Moderna that the Amsterdam-based agency has approved for a booster for adults.

"A booster dose with Covid-19 vaccine Janssen may be given after two doses of one of the mRNA vaccines authorised in the EU," it added.

It said it was closely monitoring possible side effects, which it had not currently been reported.

"The risk of thrombosis in combination with thrombocytopenia (TTS) or other very rare side effects after a booster is not known and is being carefully monitored."

Some governments are urging citizens to get booster shots as Covid cases are on the rise again globally, including in several European countries battling fresh waves.

Scientists have said that booster shots appear to protect people against the highly-mutated Omicron variant, detected in South Africa last month, which initial research suggests may be more contagious.

The EMA has so far approved four vaccines for adults in the European Union: the messenger RNA vaccines from Pfizer/BioNTech and Moderna, along with viral vector vaccines from AstraZeneca and Johnson & Johnson.