The European Medicines Agency (EMA) has expressed doubts about giving the general population a fourth COVID-19 vaccine shot, saying repeated boosters are not a "sustainable" strategy.
The EMA also said the spread of the Omicron variant of coronavirus is pushing COVID-19 towards being an endemic disease.
"While use of additional boosters can be part of contingency plans, repeated vaccinations within short intervals would not represent a sustainable long-term strategy," the EMA's head of vaccines strategy, Marco Cavaleri, told a media briefing.
He raised concerns that repeated boosters could overload people's immune systems.
"If we have a strategy in which we give boosters every four months, we will end up potentially having problems with immune response," the EMA official said.
"And secondly of course there is the risk of fatigue in the population with continuous administration of boosters."
Countries should instead start thinking about spacing out boosters at longer intervals, and, like flu jabs, synchronise them with the start of the cold season, Cavaleri said.
"With the increase of immunity in population -- and with Omicron, there will be a lot of natural immunity taking place on top of vaccination -- we will be fast moving towards a scenario that will be closer to endemicity," he added, but stressed that "we should not forget we are still in a pandemic".
The World Health Organization said on Tuesday that more than half of people in Europe were on track to catch the Omicron variant in the next two months.
WHO also warned that repeated COVID-19 boosters were not a viable strategy.