Reported breakthroughs in Covid-19 vaccine research are "encouraging", the World Health Organization's chief said on Monday, but voiced concern about surging cases and warned against complacency.

"We continue to receive encouraging news about COVID-19 vaccines," WHO chief Tedros Adhanom Ghebreyesus told a virtual press briefing.

Tedros, who has just spent more than two weeks in quarantine after coming in contact with someone with Covid-19, said he was "cautiously optimistic"  that new tools would start to arrive in the coming months.

But he added: "This is no time for complacency."

His comments came as hopes trials of a second candidate vaccine suggested it was nearly 95 percent effective against the virus.

The news from US biotech firm Moderna followed similar interim results last week for Pfizer and BioNTech's candidate vaccine.

'Playing with fire'

But WHO has warned that widespread availability of any vaccine remains a long way off, and Covid-19 cases and deaths are surging in many parts of the world.

"Those countries that are letting the virus run unchecked are playing with fire," he said. 

He voiced particular alarm about the situation in Europe and the Americas, where he said health workers and systems were being pushed to breaking point. 

"A laissez-faire attitude to the virus ... leads to death, suffering and hurts livelihoods and economies," he said. 

"The quickest way to open up economies is to defeat the virus."

The UN agency's headquarters is experiencing its own outbreak of coronavirus, with Tedros spending 17 days in quarantine -- though he said he had not developed any symptoms.

Five new cases within the same WHO team have been registered in the past week, WHO official Maria Van Kerkhove said.

"We don't know if there is an actual cluster," she said, adding that experts were trying to work out if transmission had happened on the premises.

WHO emergencies director Michael Ryan stressed that the Geneva region is experiencing "some of the most intense transmission in the world right now".

"We are human beings and we live within a society," he said.