Follow the latest updates on the Covid-19 pandemic here
The World Health Organisation on Wednesday urged "extreme caution" for countries considering lifting Covid-19 restrictions, saying high vaccination rates would not stop transmission of the virus increasing.
The UN health agency urged governments not to waste hard-won gains.
"I would urge extreme caution in the complete lifting of public health and social measures at this time, because there are consequences for that," WHO emergencies director Dr Mike Ryan said of Britain's plans to end restrictions for England on July 19.
WHO figures show that globally, after a decline in newly reported cases for seven consecutive weeks, there was a slight increase in new cases in the past fortnight.
Case numbers rose by 30 per cent in the WHO's European region over the past week.
European countries have administered 70 vaccine doses for every 100 inhabitants, AFP figures show.
"Making assumptions that transmission will not increase because we're opening up, because of vaccine, is a false assumption," Dr Ryan said.
"Transmission will increase when you open up because we don't have vaccines in everybody ... and we're still not sure to what extent vaccination protects against the ability to be infected or have onward transmission.
"With increased transmission in the community, we then put our most vulnerable at risk again."
British Prime Minister Boris Johnson on Monday revealed plans to lift most of England's coronavirus restrictions, including face masks and social distancing.
Mr Johnson appealed for people to take responsibility without legal requirements.
Dr Ryan said he did not want to comment on specific countries, but "the idea that everyone is protected ... and everything goes back to normal, I think right now is a very dangerous assumption anywhere in the world.
"We would ask governments to be really careful at this moment not to lose the gains we've made; to open up very carefully.
"I would hope that in the European environment we won't see a return to the overwhelmed hospitals and the exhausted health workers. But that's not a given."
Dr Ryan said countries lifting restrictions must follow the data and give themselves the option of returning to restrictions if necessary.
Opening up with low vaccine coverage and variants circulating "is a real toxic mixture for your hospitals filling up again and this is something that must absolutely be avoided", he said.
Mr Ryan flatly rejected that it would be better to lift restrictions and take the risk of a new wave of infections in the summer, rather than winter.
He said that reasoning was "logic that has proven its moral emptiness and its epidemiologic stupidity".