A third-dose booster shot of the COVID-19 vaccination is a way to keep the most vulnerable safe and "not a luxury," the World Health Organization said on Monday.

The WHO said earlier this month that data did not indicate a need for booster shots while topping up already fully vaccinated people would further widen a vaccine-availability gap between rich and lower-income countries.

"A third dose of vaccine is not a luxury booster (that is) taken away from someone who is still waiting for a first jab. It's basically a way to keep the most vulnerable safe," Hans Kluge, head of WHO Europe, told a press briefing.

"We have to be a little bit careful with the booster shot, because there is not yet enough evidence," he said.

"But more and more studies show that a third dose keeps vulnerable people safe, and this is done by more and more countries in our region," he said.

Kluge urged European countries with excess vaccines to share them with other countries, particularly those in Eastern Europe and Africa.

An increase in COVID-19 transmission rates across Europe over the last two weeks, combined with low levels of vaccination in some countries, was "deeply worrying," he said.