China is experiencing a huge surge in cases of the virus since loosening hardline restrictions which involved large-scale lockdowns.
Shanghai, a city of 25 million people, in April ordered a gruelling two-month lockdown after 600,000 residents were infected and many were hauled to mass quarantine centres.
Chen Erzhen, vice president at Ruijin Hospital and a member of Shanghai's Covid expert advisory panel, estimated that the majority of the city's 25 million people may have been infected.
“Now the spread of the epidemic in Shanghai is very wide, and it may have reached 70 per cent of the population, which is 20 to 30 times more than [in April and May],” Mr Erzhen told the People's Daily newspaper.
The Omicron variant is spreading rapidly across the city and experts predict infections there will peak in early 2023.
Beijing has admitted the scale of the outbreak has become “impossible” to track following the end of mandatory mass testing last month.
The National Health Commission has stopped publishing daily nationwide infection and death statistics.
That responsibility has been transferred to the Chinese Centre for Disease Control and Prevention (CDC), which will only publish figures once a month after China downgrades its management protocols for the disease on January 8.
Last month, a few local and regional authorities began sharing estimated daily infection totals as the scale of the outbreak remained unclear.
Disease control authorities in the wealthy coastal province of Zhejiang said on Tuesday that the number of new cases had jumped one million in the past few days, and “the epidemic is expected to enter a peak plateau in January”.
The Zhejiang cities of Quzhou and Zhoushan said at least 30 per cent of the population had contracted the virus.
The eastern coastal city of Qingdao estimated about 500,000 new daily cases, and the southern manufacturing centre of Dongguan up to 300,000.
Officials in the island province of Hainan estimated on Friday that the infection rate there had surpassed 50 per cent.
But top health official Wu Zunyou said on Thursday that the peak had passed in the cities of Beijing, Chengdu and Tianjin, with Guangzhou city officials saying the same on Sunday.
Lunar New Year travel
China's biggest holiday, Lunar New Year, is expected to see 5.5 million people use China's rail system, state broadcaster CCTV has said.
Travel has been severely restricted for the event for the last three years, but as people plan to head from cities to their home villages to celebrate the January 21 holiday, fears of more infections are mounting.
In an interview with state broadcaster CCTV on Monday, National Health Commission (NHC) official Jiao Yahui admitted that dealing with the expected peak in rural areas would be an "enormous challenge".
"There may be a retaliatory surge of urban residents into the countryside to visit their relatives, so we are even more worried about the rural epidemic."
Chinese authorities will brief the World Health Organisation on the situation on Tuesday. It reversed Covid controls on December 7 after protests over its “zero-Covid” approach in the strongest show of public defiance in decades.
The US, France, and other countries now require Covid tests on travellers from China, while Belgium said it would test wastewater from planes for new variants.
European Union health officials will meet on Wednesday to discuss a co-ordinated response.
China said the requirements lack scientific basis and are unreasonable.
Mao Ning, a spokeswoman for the Chinese foreign ministry, said China is “firmly opposed to such practices” and will take corresponding measures accordingly.