The transmission capability of asymptomatic patients of COVID-19 is about one-third of that of symptomatic ones, said Wu Zunyou, chief epidemiologist at the Chinese Center for Disease Control and Prevention on Thursday, citing a study recently published in the Chinese Journal of Epidemiology.

Asymptomatic patients refer to those who tested positive for the disease but show no clinical symptoms.

In the study, researchers from Ningbo Center for Disease Control and Prevention observed close contacts of confirmed patients and of asymptomatic patients to see the morbidity in both groups.

They found that one confirmed patient can spread the virus to nearly three people on average, while an asymptomatic patient can spread to less than one other person.

When considering the whole group surveyed, only 4.4 percent of cases in the study were transmitted by asymptomatic patients, which means "the asymptomatic patients play relatively small role in spreading the epidemic," said Wu, adding that the study by Ningbo was only conducted at a regional level.

Other provinces have also done some research.

A larger-scale study in a southern province showed that seven of over 1,500 people who had close contact with asymptomatic patients became ill, said Wu. But further analysis found that the seven cases had contact not only with the asymptomatic patients, but also with confirmed patients.

"That is to say we cannot attribute the infection to the asymptomatic," Wu said, cautioning that if not investigated carefully, these cases with exposure to both symptomatic and asymptomatic patients, may lead to an exaggerated assessment of the transmissibility of the asymptomatic.

More research and observation is needed to gain a better understanding as there is limited data available on the infective period, transmission capacity and transmission contribution of the asymptomatic, he said.

The expert also informed that all asymptomatic patients must have been in contact with a confirmed patient or have traveled to an epidemic area. No asymptomatic infections will occur for no reason in an area that went through an outbreak but has been clear of new confirmed cases for more than two weeks.

"The current measures we take can detect and control the asymptomatic in a timely manner," said Wu. "There is low possibility that they would cause an epidemic, or scale up to a social-level spread."