Asymptomatic infections are small percentage of novel coronavirus totals

Asymptomatic infection of the novel coronavirus, which has caused increasing concern among the public, is still a very small proportion among total infections, and could be effectively detected by nucleic acid testing, a senior testing expert in Shanghai said.

"The test result is a numerical number and it doesn't have anything to do with whether the patient will eventually show symptoms or not," said Li Chongshan, deputy director of the Division of Microbiology under the Shanghai Municipal Center for Disease Control and Prevention.

Amid the rising number of asymptomatic infections, China has scaled up screening of such cases and expanded testing to close contacts of infected patients in an effort to prevent a possible domestic rebound of the outbreak.

Seen from Shanghai's data and that of the whole country, patients with asymptomatic infections who did not show any symptoms throughout the 14-day incubation period or even longer, only accounted for between 1 and 5 percent of the total number of patients so far, Li told China Daily in an exclusive interview on Tuesday.

"A case that didn't show any symptoms of the novel coronavirus when testing positive for the virus would usually develop symptoms in several days. That means they were in the incubation period of the virus when taking the test," he said.

Medical experts said that in most cases, a person can test positive for the virus through a nucleic acid test two to three days after getting infected.

The Shanghai CDC Division has been responsible for producing lab test results for inbound passengers arriving in Shanghai who showed fever or respiratory symptoms. It also is responsible for suspected cases discovered through epidemiological investigations at customs since the municipality beefed up measures to curb the spread of the virus from overseas in early March. About 200 to 300 samples are being sent for tests at labs there every day.

All the other samples from inbound travelers were sent to the 16 district CDCs in the municipality according to their residential addresses. Shanghai started on March 6 to require travelers from four countries severely affected by the outbreak to be subject to epidemiological investigation at customs and a 14-day quarantine. The scope has since been expanded gradually, and as of March 23 all inbound passengers must take the nucleic acid testing.

Li said 37 lab workers are divided into groups and working around the clock in the labs to ensure the test results are produced efficiently and accurately.

Also, there is no false-positive or slightly positive COVID-19 case in the final clinical determination, he said.

"There are actually some test samples, of which the result of the test lasting six to eight hours, fall into the grey area between positive and negative, and we'll turn to a testing kit from another producer for another test. If both tests show the same result, we'll categorize it as a positive case, judging from the lab tests," he said, adding that they regularly use testing kits from three producers.

If the second test produces a negative result, they will request another sample, a swab at the person's throat, in 24 hours, for another test. If the third test result is positive or between positive and negative, they will determine this as a positive case. Otherwise, it will be deemed as a negative case, he said.

Till now, they have not missed one single positive case or mistaken a negative case as positive, according to Li.