1 in 5 asymptomatic patients show symptoms in China, but experts play down infection risk
Concerns rose after about one fifth of asymptomatic infections turned into confirmed patients recently in China.
China has so far reported a total of 6,764 asymptomatic infections, of whom 1,297 were later classified as confirmed patients, according to the National Health Commission on Wednesday.
On Wednesday alone, 64 silent coronavirus carriers were discovered across the Chinese mainland, bringing the total number of silent carriers under medical observation to 1,032. On that day, six of the silent carriers became confirmed cases.
Asymptomatic infection remains a challenge, Zeng Guang, chief epidemiologist of the Chinese Center for Disease Control and Prevention, told the Global Times.
Symptom-free patients are not considered to be as contagious as confirmed ones, and China will bring asymptomatic cases under control, Zeng said, noting "there's no need to worry too much about the matter."
So far, about 380,000 residents in Wuhan have received nucleic acid tests, and fewer than eight cases out of every 10,000 individuals were found to be asymptomatic infections, according to Hu Yabo, the deputy mayor of Wuhan.
Each asymptomatic infection case undergoes an epidemiological investigation in the same way as a confirmed case, and random nucleic acid tests were also given to their close contacts. None of them developed into confirmed COVID-19 patients, the deputy mayor said.
As the proportion of asymptomatic infections in Wuhan, the region hit hardest by the epidemic, is less than eight out of 10,000, the proportion in other regions should be lower, Wang Peiyu, deputy head of Peking University's School of Public Health, told the Global Times.
A medical worker takes blood sample for a COVID-19 test of an overseas Chinese student returning home at a quarantine location in Taiyuan, North China’s Shanxi Province on March 17. Photo: cnsphoto
The main concern for China is not silent virus carriers, but infections from abroad, as other countries continue to see growing numbers of people being infected, while the origin of COVID-19 is still unknown, Wang noted.
Wang Hui, deputy head of Wuhan's Wuchang district health commission, told the Global Times that they have found more than 100 asymptomatic patients in her district, but added there were no asymptomatic patients who later showed symptoms, saying such patients are rare exceptions.
Wang said there's no need to be too concerned about a new wave of infections caused by asymptomatic patients. "We will fix the potential hazard with long-term follow up procedures, regular nucleic tests for those patients and other examination measures."
Experts are still in the process of getting a deeper understanding of asymptomatic cases. Different regions and provinces may have different ratios of silent virus carriers, and epidemiological investigations are still underway, experts said.
To further find asymptomatic infections and their ability to infect, China has been conducting an epidemiological survey in Wuhan and nine other provinces and cities, including Beijing, Shanghai, and South China's Guangdong Province.
Wuhan's ongoing epidemiological investigation is helping with tracing asymptomatic patients. Those asymptomatic patients did not just come out of the blue; some are close contacts of confirmed patients, according to their partial analysis, Wang told the Global Times in Wuhan.
In order to prevent the emergence of large numbers of patients without symptoms, Wuchang district plans to conduct long-term follow-up and callback procedures on certain groups of people - confirmed cases, suspected cases and asymptomatic patients, according to Wang.
Regarding the infectivity of silent coronavirus carriers, a study by the Ningbo Center for Disease Control and Prevention showed that a confirmed patient can infect three other people while asymptomatic patients can only infect less than one person.
As the infectivity of an asymptomatic patients is only a third of that of a confirmed patient, the total infections that result from asymptomatic patients account for less than 5 percent of total COVID-19 patients, the study found.
As with medical quarantine measures for confirmed patients, China requires silent coronavirus carriers to undergo two nucleic acid tests after 14 days of quarantine. Only when both results come back negative can they be discharged from quarantine, when then they will undergo another 14 days of medical observation, the National Health Commission said.
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, will they be categorized as a confirmed case, the standard says.