Fifteen domestic cases of people infected with COVID-19 variants of concern have been detected in the Chinese mainland since early December, experts said.

However, the strains, including the highly contagious XBB.1.5 and BQ.1 variants, have not spread widely and are highly unlikely to cause major outbreaks in the first half of this year, they said.

The mainland has been watching for emerging variants with high transmissibility or immunity evasiveness. From December to Thursday, samples from over 16,900 COVID-19 cases had been collected and their genome sequences analyzed, the Chinese Center for Disease Control and Prevention said on Saturday.

Among the 15 infections are one XBB.1.5 case and one BQ.1 reported this month, data from China CDC shows.

The domestic emergence of the two strains has drawn attention in the medical community and among the public because they have spread rapidly in a number of countries in recent months. In China, the dominant strains are BA.5.2 and BF.7.

Chen Cao, a researcher at China CDC, said BQ.1 was first detected in June in Nigeria and spread to Europe and the United States in September, followed by Japan and Singapore in mid-October.

The XBB.1.5 strain began to spread widely in the US in November. By Jan 2, it had been reported in at least 25 countries and regions, according to China CDC.

Chen said the majority of people in China have recently been infected and have a high level of neutralizing antibodies and immunological memory cells in their bodies.

"Under such a circumstance, even if a new Omicron strain surfaces, the possibility of it sparking large-scale domestic outbreaks is low," he said. "However, vulnerable and high risk groups, such as people age 65 and above and people with preexisting illnesses could get infected again."

Chen said that there is no need to panic, but the threat should not be disregarded and people should take precautions to protect their health.

Li Tongzeng, a doctor at Beijing Youan Hospital, affiliated to Capital Medical University, said during an interview with Chinanews.com that China's herd immunity is strong, with only sporadic infections recently.

"The risk of a second infection will increase when the number of patients infected with novel strains grows," he said.

Over time, protection against new variants could diminish. However, it will remain effective, so the risk of a fresh wave of infections is very small, Li added.

Zeng Guang, former chief epidemiologist at China CDC, estimated less than 10 percent of the total population is still susceptible to the disease, as the majority of people had been infected and gained immunity during the recent outbreak, and over 3.4 billion COVID-19 vaccination doses had been administered nationwide.

"The immunity barrier at present is quite robust," he said.

Zeng said that no variant capable of breaking the immunity barrier has emerged. However, close monitoring of new strains should be implemented and future vaccination strategies should be formulated.

According to China CDC, from Feb 10 to Thursday, the mainland reported 98 COVID-related deaths at hospitals, including one caused by the virus alone. The remainder were caused by a combination of COVID-19 and other preexisting illnesses.

That toll compares with the 912 deaths reported during the previous seven-day period from Feb 3 to 9.

On Thursday, there were 20,000 people with COVID-19 in hospitals, down 98.8 percent from the peak recorded on Jan 5.