Recent clusters of coronavirus infections in China were mainly caused by inbound visitors and imported goods, and port cities that are at constant risk of exposure to such sources are prone to see the resurgence of the disease, a senior Chinese official said on Wednesday. 

Feng Zijian, a deputy director general of the Chinese Center for Disease Control and Prevention (China CDC), made the remarks at a media briefing on Wednesday. 

He said that according to investigations, the recent outbreaks in places including North China's Hebei Province were not related to previous domestic cluster infections, but caused by newly imported sources of the virus.

Although the epidemiological investigations have not proved that airports or ports were directly related to the infections, fresh cluster infections in Hebei and Beijing occurred near airports, which have raised some concerns that possible loopholes in airports and port cities may contribute to these outbreaks. 

Ongoing cluster infections in Hebei came from abroad, and the Shijiazhuang Zhengding International Airport was one of the airports for overseas travelers entering China, Shi Jian, an official of the Hebei CDC, told China Central Television. Shi noted that health authorities are considering imported personnel as the possible source of the outbreak, although no conclusions have been reached.

A sudden spike in COVID-19 cases at the end of December in Beijing's Shunyi district, where the Beijing International Airport is located, was caused by an imported case from Indonesia. The Indonesian patient came to Beijing on December 10 from East China's Fujian Province.

Jin Dongyan, a biomedical professor at the University of Hong Kong, told the Global Times that recent outbreaks involving hundreds of infections were most likely caused by travelers entering China, as cluster infections caused by imported goods generally result in smaller-scale outbreaks.

 Air crew members and staff at domestic airports are among the first groups of people those overseas travelers come into contact with, and after travelers leave the airport, drivers who take them to mass quarantine facilities must protect themselves, Jiang Qingwu, a Shanghai-based epidemiologist, told the Global Times on Wednesday.

"Those protective measure are not difficult technically, but we have to make sure they are strictly implemented, as any carelessness may result in serious consequences," Jiang said.

Experts said that management at Chinese airports is the strictest worldwide, although it's nearly impossible to completely protect staff at the airports against imported cases of the virus.

A Chinese health expert who requested anonymity said that port cities may witness a rebound of the virus, but that does not mean that the management of airports or ports should be blamed. 

There have been more cases with super-long incubation periods detected in China, and these cases have posed great challenges in epidemic prevention and control for airports and ports, experts said. 

The initial case in the Shunyi cluster of infections had a positive result from a serum antibody test 16 days after he finished a 14-day quarantine in Fujian. 

Chinese experts suggested regular nucleic acid testing and antibody tests for people associated with airports, and stricter follow-up health checks for travelers who completed their quarantine periods to eliminate the risks of virus importation.