The peak of China's Covid-19 wave is expected to last two to three months, and will soon swell over the vast countryside where medical resources are relatively scarce, a top Chinese epidemiologist has said.

Beijing last month abruptly abandoned the strict anti-virus regime of mass lockdowns that fuelled protests across the country in late November, and finally reopened its borders last Sunday.

The dismantling of restrictions unleashed the virus on the nation's 1.4 billion people, more than a third of whom live in regions where infections are already past their peak, according to state media.

But the worst of the outbreak is not yet over, warned Zeng Guang, the former chief epidemiologist at the Chinese Center for Disease Control and Prevention, according to a report published in local media outlet Caixin.

"Our priority focus has been on the large cities. It is time to focus on rural areas," Zeng was quoted as saying.

He said a large number of people in the countryside, where medical facilities are relatively poor, are being left behind, including the elderly, the sick and the disabled.

Hundreds of millions of people travel to their home towns for the Lunar New Year holidays, which officially start on January 21.

Authorities have said they are making efforts to improve supplies of antivirals across the country. Merck & Co's Covid treatment molnupiravir is expected to be made available on the mainland from Friday.

The World Health Organization this week also warned of the risks stemming from holiday travelling.

The UN agency said China was heavily under-reporting Covid deaths, although it is now providing more information on its outbreak.

The Foreign Ministry said the country's health officials have held five technical exchanges with the WHO over the past month and have been transparent.

Health authorities have been reporting five or fewer deaths a day over the past month but long queues have been seen at many funeral homes.

The country has not reported Covid fatalities data since Monday. Officials said in December they planned to issue monthly, rather than daily updates, going forward.

Mainland health authorities have reported just over 5,000 Covid deaths since the pandemic began, one of the lowest fatality rates in the world. (Reuters)