Health officials believe China's dynamic zero-COVID-19 strategy remains effective in controlling coronavirus outbreaks, as Omicron infections surfaced in major cities including Beijing and Shanghai over the weekend.

However, they added that research on various COVID-19 variants, including Omicron, will be intensified, and risk evaluations done quickly to deal with mounting challenges.

Beijing reported its first locally transmitted Omicron case on Saturday. The patient lives in the capital's northwestern Haidian district and had no travel history outside the city in the past 14 days, according to the municipal government. Investigations are continuing, including genome sequencing and checking the infected woman's movements in Beijing.

Authorities have locked down the woman's residential compound and workplace. By Saturday evening they had traced 15 close contacts of the patient. As she had visited a number of public places such as shopping malls and restaurants, mass nucleic acid testing is underway, health authorities said over the weekend.

Beijing authorities have also tightened controls on inbound travelers. From Saturday, they will be required to take a nucleic acid test within 72 hours of arriving in the capital.

In Shanghai, three asymptomatic COVID-19 infections were reported on Saturday. Officials said that they were all linked to Omicron infections detected on Thursday, and all three tested positive during quarantine. In Guangdong province, Zhuhai and Zhongshan have registered 12 locally transmitted Omicron infections since Thursday, according to health authorities.

The locally transmitted Omicron cases have emerged as China prepares for mass travel during the Spring Festival holiday and ahead of the Beijing 2022 Winter Olympics, which are due to start on Feb 4.

He Qinghua, an official at the National Health Commission's Disease Prevention and Control Bureau, said Omicron has spread to over 150 countries and regions worldwide.

"The risk of the strain being transmitted into our country via people or objects is getting larger," he said at a news conference on Saturday.

In China, 14 provincial-level regions have reported imported Omicron infections, He said. He added that the new strain also caused outbreaks in Tianjin and Henan province, which has been brought under control.

Despite the complicated situation, He said the epidemic situation in China is largely stable and the country's zero-COVID-19 strategy of eliminating new local outbreaks as soon as possible remains effective.

To prevent imported cases, He said China is implementing rigorous management of incoming passengers and high-risk workers who come into contact with travelers or items from overseas. He added that random sample taking as well as sanitizing of international aircraft, luggage and cargo has been stepped up.

To avert domestic flare-ups, He said monitoring and early warning systems targeting key groups and venues are in place. Meanwhile, mass vaccinations and campaigns to guide the public on personal protection against COVID-19 will continue to be rolled out.

As of Friday, China had fully vaccinated over 1.22 billion people, or nearly 86.6 percent of the total population, the commission said.

He added that the commission will keep a close eye on global transmission of different COVID-19 variants, including Omicron, and intensify research on their transmissibility and pathogenicity, as well as their effects on vaccines.