Health experts have warned that the increase in covid-19 cases in China, as the country abandons restrictions, could be fertile ground for new variants to emerge.

Beijing announced on Wednesday the end of mandatory quarantines on arrival in the country from January 8, in the last vestige of the "zero covid" policy that has kept China closed to the world for almost three years since the pandemic began.

Although the Chinese government has stopped publishing the number of daily cases, officials in several cities have estimated that hundreds of thousands of people have been infected, while hospitals and crematoria are overwhelmed across the country.

With the virus now free to circulate among almost a fifth of the world's population, many countries and experts fear that China is becoming a breeding ground for new variants.

Each new infection increases the chances of the virus mutating, said Antoine Flahault, director of the Institute of Global Health at the University of Geneva.

"The fact that 1.4 billion people are suddenly exposed to SARS-CoV-2 obviously creates favorable conditions for the emergence of variants," he told the France-Presse news agency (AFP).

Bruno Lina, professor of virology at the French University of Lyon, told La Croix that "given the intense circulation of the virus, and therefore the increased risk of mutations, a potential set of viruses could emerge from China.

Soumya Swaminathan, who until November was chief scientist at the World Health Organization (WHO), also said that a large part of the Chinese population is vulnerable, partly because many elderly people had not been vaccinated.

"We need to be on the lookout for any emerging variants of concern," he told the Indian Express.

In response to the increased outbreak of cases, Japan, India and the United States will require PCR tests for all passengers coming from China, a measure that Antoine Flahault said could be a way of circumventing any delays in information coming from Beijing.

"If we can sample and sequence all the viruses identified in all travelers in China, we will know almost immediately if new variants emerge and spread" in the country, Flahault said.

Xu Wenbo, head of the virus control institute at the Chinese Center for Disease Control and Prevention, said that hospitals across the country will collect samples from patients and enter the sequential information into a new general database, allowing authorities to monitor new strains in real time.

More than 130 new omicron sub-variants have been detected in China in the last three months, Xu said last week.

These include XXB and BQ.1, but BA.5.2 and BF.7 remain the main omicron strains detected in China, the Chinese official said.

A "soup" of more than 500 new sub-variants of omicron has been identified in recent months, emphasized Antoine Flahault.

"All variants, when they are more transmissible than previously dominant variants - such as BQ.1, B2.75.2, XBB, CH.1 or BF.7 - definitely pose threats, because they can cause new outbreaks," said the epidemiologist.

"Today, none of these variants appear to present specific new risks of more serious symptoms, but this could happen in the near future," he added.