The agency said it was too early to tell what impact the new sublineage of sub-variant BA1.75, which was detected in India and spread to at least 10 countries, would have.
“It's still too early to know if this sub-variant has properties of additional immune evasion or indeed of being more clinically severe. We don't know that. So, we have to wait and see," Soumya Swaminathan, chief scientist at the WHO, said.
The WHO said Europe is the current centre of the resurgence, with more than 80 per cent of current Covid-19 infections, as more people mix at large-scale events and travel.
WHO director general Tedros Adhanom Ghebreyesus said Covid-19 infections rose 30 per cent globally in the past two weeks, with the Omicron subvariants BA.4 and BA.5 driving the increase in Europe and the US.
"Testing has reduced dramatically in many countries … [and] this obscures the true picture of an evolving virus and the real burden of Covid-19 disease globally,” he said.
"New treatments - especially promising new oral antivirals - are still not reaching low and low middle-income countries, depriving whole populations that need them."
And while death rates are lower than at the height of the pandemic, Dr Tedros cautioned that "each wave of the virus leaves more people with long-Covid or post-Covid conditions [which] obviously impacts individuals and their families, but it also puts an extra burden on health systems, the wider economy and society at large."
Executive director of the WHO’s Health Emergencies Programme, Michael Ryan, said that while Europe was the epicentre, the sublineage would probably spread elsewhere.
“We are seeing a much more intense wave of the disease passing through Europe again, and we will see it happen elsewhere — we are already seeing it in South East Asia and in the eastern Mediterranean region as well,” he said at a briefing on Wednesday.
The increase in cases in Europe during the summer — when transmission should be more difficult because people are outside — results from people swapping one kind of mixing for another, such as attending large concerts and travelling more, Mr Ryan said.
The new wave of infections isn’t yet leading to a surge in intensive care admissions or a serious increase in the number of deaths, as a result of vaccine protection.
Last week, Britain reported a 32 per cent rise in infections and said hospital admissions are climbing, with intensive care cases spreading among older age groups. The rise in hospitalisations is still far below previous waves, and widespread vaccination has made the pandemic far less deadly.
The WHO said countries should ensure their “immunological walls” remained strong, provide booster shots as necessary to vulnerable individuals, maintain surveillance measures, introduce antivirals and use tried-and-tested countermeasures to prevent transmission.
Dr Tedros said that while any resurgence in cases has to be taken seriously, the world is in a better situation to fight the virus now than it was in 2020.
“We won’t be the hostage of the virus like we have in the past two years,” he said. “We know the virus, we have better tools to fight it.”
Separately, Dr Tedros said the organisation remained concerned about the spread of monkeypox — with more than 6,000 cases in 58 countries — and will reconvene the agency’s emergency committee within the next three weeks to determine if the outbreak should be considered a public health emergency of international concern.