China is tracking asymptomatic cases. Why does that matter?

China began to release data on asymptomatic coronavirus patients last week, a move experts say will help other countries respond to the pandemic and provide crucial insight into how the virus is spread.

"We have been basing a lot of our models and our predictions off the Chinese data because it was the first major outbreak," Nadia Abuelezam, an epidemiologist at Boston College's Connell School of Nursing, told NBC News.

With the addition of asymptomatic patients -- those infected but showing no symptoms of the disease -- raising the count, she said, "this changes the potential dynamics of the models."

Scientists are still trying to determine the risk posed by asymptomatic cases.

Abuelezam said while people displaying symptoms spread the virus through droplets more likely to be projected while coughing, someone who is not showing the symptoms but still has the virus could transmit the disease in the same way.

A study out of Singapore released Wednesday estimated the transmission rate by healthy-seeming individuals was significant. It found around 10 percent of new infections could be caused by asymptomatic patients.

As a result, the U.S. Centers For Disease Control and Prevention changed its guidance on preventing infections and warned that anyone could be a carrier.

In China, where the coronavirus originated, the total number of asymptomatic cases recorded since the outbreak began has not been released, but Wu Zunyou, the country's chief epidemiologist, said such cases accounted for 4.4 percent of the total confirmed patients.

A total of 81,554 people in China have been infected with COVID-19 and 3,312 people have died from the disease, according to the country's National Health Commission.

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However, asymptomatic people do not appear to be major contributors to the spread of the virus, compared with people who are showing symptoms, Wu said during a State Council Information Office briefing last week.

Cautioning that more research was needed, he said: "If the investigation is not carried out carefully, it will exaggerate the transmission ability of asymptomatic infection."


Asymptomatic patients are nonetheless required to go into mandatory 14-day quarantine as soon as they are identified, according to the National Health Commission.

On Monday, Chinese Premier Li Keqiang called for more efforts to trace, monitor and treat those infected with COVID-19, even when showing no symptoms, to further reduce the spread of the disease.

His comments came after it was decided at a meeting with his team managing the response to launch an investigation into asymptomatic cases in the city of Wuhan where the outbreak began and Hubei province where the city is based.

Lockdown restrictions are slowly being lifted in the city but fears of a second wave of the disease are running high.

Li's team said they intend to publicize their findings and provide information on targeting efforts to contain the coronavirus. Experts have called on all countries to share as much data as possible.

Image: Disinfection efforts in Wuhan  (AFP - Getty Images)
Image: Disinfection efforts in Wuhan (AFP - Getty Images)

"Any additional information can help us gain a better picture of what’s going on transmission-wise, what’s going on epidemiologically," Abuelezam said.

"If it's the case that there are many more asymptomatic people and they are transmitting it to other people, then that should motivate us to expand testing," she added, pointing out that it could change government strategies on mitigating the spread of the disease.

Dr. Tim Spector, professor of genetic epidemiology at King’s College London, agreed that the more information available, the better.

"We need to broaden our scope of data and not be so arrogant to think we really understand what's going on based on the first two common symptoms that's coming out of China," said Spector, who is part of a team collecting data from over 2 million people in the United Kingdom using a COVID-19 symptom tracking app that is also available in the United States.


Every discovery can influence not just how governments are reacting to the virus, but how doctors are approaching it in their clinics, Spector added.

A trend in symptoms in Britain can inform whether a doctor in another country suspects a patient has the virus if they're presenting similar symptoms, he said.

Both Abuelezam and Spector agreed that missing data and the different ways that countries recorded data were hampering research.

Testing in many jurisdictions, including the U.S., is limited to people who are already sick and seeking care — not those who may feel fine or treating their symptoms at home, Abuelezam said.

Then, there's political influence affecting the data, as some countries are "trying to pretend their death rates are better than other people’s," Spector said.

"We need to move away from that," he added. "It’s very much by being open and sharing data that we can make big progress here."