"This is a wake-up call to recognise that there are ups and downs in clinical development and that we have to be prepared," Soumya Swaminathan told a virtual briefing from Geneva.

"We do not have to be discouraged. These things happen."

Governments are desperate for a vaccine to help end the COVID-19 pandemic, which has caused more than 900,000 deaths and global economic turmoil, and the WHO had flagged AstraZeneca's, being developed with Oxford University, as the most promising.

However, the drug maker suspended late-stage trials this week after a participant in Britain suffered from neurological symptoms.

"It's a race against this virus, and it's a race to save lives. It's not a race between companies, and it's not a race between countries," added WHO's head of emergencies Mike Ryan.

More than 27.95 million people have been reported infected globally, according to a Reuters tally.

WHO epidemiologist Maria Van Kerkhove said a combination of factors is helping reduce death rates in Europe, including finding cases earlier and better clinical care.

"We are in a better position to prevent the virus from infecting vulnerable populations," she said, cautioning, however, that the disease's long-term effects were still not known.

WHO General Director Tedros Adhanom Ghebreyesus, who on Thursday raised his fundraising plea to $US38 billion ($A52 billion) for the agency's ACT Accelerator program to fight COVID-19, called for global solidarity.

"What worries me the most is what I have been saying all along: a lack of solidarity," Tedros said.

"When we are divided, it is a good opportunity for the virus."

Australian Associated Press