Photo:China News Service

Countries and regions that are still at the early stages of the COVID-19 pandemic are urged to beware of "silent virus carriers" and expand screening on those who have close contact with the infected even if they have no symptoms. Chinese experts cautioned on Monday that asymptomatic patients are also contagious and could have contributed to the pandemic spreads.

As the epidemic prevention and control in China has entered a phase of preventing the second wave of outbreak and the pandemic abroad is deteriorating, those patients who showed no symptoms but tested positive for COVID-19 have gradually entered the public spotlight. 

Several cities in China have recently found asymptomatic patients including one in Wuhan and two cases in Xiangyang, Central China's Hubei Province. 

Those silent carriers could be an important source of infection, warned Hu Bijie, director of the Department of Infectious Diseases from Zhongshan Hospital affiliated to Fudan University, suggesting other countries such as the US and European nations should strengthen efforts to find and quarantine those patients. 

"It could be an important reason why the COVID-19 pandemic spreads in those countries in a quick speed," Hu told the Global Times on Monday. 

But these asymptomatic patients will not have impact on the overall epidemic situation in China, Zhang Wenhong, director of the Department of Infectious Diseases from Shanghai's Huashan Hospital said. 

 "Basically, patients would show symptoms within two weeks, if not, three or four weeks. Most regions in China have seen zero new confirmed cases for at least two weeks and in some cities three weeks, such as Shanghai, which means asymptomatic patients are already non-existent," Zhang told media. 

Asymptomatic patients generally appear in the period of strong epidemics. The longer the duration of zero new patients, the fewer asymptomatic carriers there will be, Zhang explained.

But countries such as the US and Italy simply do not test people with no symptoms. In contrast, China imposes quarantine measures on those asymptomatic cases for 14 days once they test positive for the virus , even though they were not counted as "confirmed patients" in China.

Facing increasing overseas arrivals in China and the expanding pandemic abroad, Wuhan virologist Yang Zhanqiu suggests testing all passengers with or without symptoms.

Referring to the number of asymptomatic patients in China, Yang told the Global Times that this group of patients only accounts for a minor proportion. 

"Previous studies on other coronavirus infections showed that the percentage of asymptomatic patients is about 1 percent to 3 percent of the total, or at maximum 5 percent. So presumably, COVID-19 has a similar percentage of asymptomatic infections," Yang said. 

A patient usually develops symptoms in five days but only in extremely rare cases their incubation period surpasses 30 days, such as the two confirmed cases, from Dandong, Northeast China's Liaoning Province who only showed symptoms after more than a month, the local government said in early March. 

Regarding the degree of infectivity, the subject remains controversial. A recent study conducted by the Guangzhou Eighth People's Hospital and the Public Health School of Hong Kong University showed the most infectious moment of the COVID-19 patients is likely to be just before they show symptoms.

But Yang said generally speaking their infectivity is less than those showing symptoms, who are still the main source of infection. "As spring comes, the climate is warmer. This makes it difficult for the virus to survive, therefore the contagiousness of these asymptomatic patients becomes weak accordingly." 

Asymptomatic patients are defined as those who have no clinical symptoms but test positive for COVID-19, according to the standard issued by the National Health Commission. 

Only when they later show clinical symptoms, such as fever or respiratory symptoms and changes in the quantity of white blood cells, they will be categorized as a confirmed case, the standard says.