Beijing is facing a surge in severe COVID-19 cases amid concerns over the potential toll and knock-on effects for the world economy after the surprise ditching of China's tough virus policies.

Following widespread protests, the country of 1.4 billion people this month began dismantling its "zero-COVID" regime of lockdowns and testing that had largely kept the virus away for three years, at great economic and psychological costs.

The easing coincided with an increase in COVID cases that experts say will likely gather pace through winter, with projections suggesting China could face more than one million deaths next year.

China, which uses a narrow definition of what can be classified as a COVID-19 fatality, reported no new COVID deaths for December 20, compared with five the previous day.

The nation's overall fatalities since the pandemic began were revised to 5241 after removing one death in Beijing.

That number might rise sharply in the near future, with state-run Global Times citing a leading Chinese respiratory expert predicting a spike in severe cases in the capital in the coming weeks.

"We must act quickly and prepare fever clinics, emergency and severe treatment resources," Peking University First Hospital respiratory expert Wang Guangfa told the newspaper.

Severe cases rose by 53 across China on Tuesday, versus an increase of 23 the previous day.

China does not provide absolute figures for severe cases.

Wang expects the COVID wave to peak in late January, with life likely to return to normal by the end of February or early March.

Amid doubts over what is a low COVID death toll by global standards, China's National Health Commission on Tuesday clarified only people whose death is caused by pneumonia and respiratory failure after contracting the virus are classified as COVID deaths.

Benjamin Mazer, an assistant professor of pathology at Johns Hopkins University, said that classification would miss "a lot of cases" especially as people who are vaccinated, including with the Chinese shots, are less likely to die of pneumonia.

Blood clots, heart problems and sepsis - an extreme body response to infection - have caused countless deaths among COVID patients around the world.

"It doesn't make sense to apply this sort of March 2020 mindset where it's only COVID pneumonia that can kill you when we know that in the post-vaccine era, there's all sorts of medical complications," Mazer said.

The NHC also played down concerns raised by the United States and some epidemiologists over the potential for the virus to mutate, saying the possibility of new strains that are more pathogenic is low.

Several leading scientists and World Health Organization advisors told Reuters it might be too early to declare the end of the global COVID pandemic emergency phase because of a potentially devastating wave to come in China.

Last week, WHO chief Tedros Adhanom Ghebreyesus said he was "hopeful" of an end to the emergency next year.

The US on Tuesday indicated it stands ready to assist China with its outbreak, warning an uncontrolled spread in the world's second-largest economy could have implications for global growth.

A major near-term concern for economists is the impact a surge in infections might have on factory output and logistics as workers and truck drivers fall ill.

The World Bank on Tuesday cut its China growth outlook for this year and next, citing the abrupt loosening of COVID measures among other factors.