New NIH Study: Transmissibility of COVID-19 by Asymptomatic Carriers Is Weak

(AP Photo/Mark Schiefelbein)
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Employees work in a research and development lab of Beijing Applied Biological Technologies, a firm which is developing COVID-19 molecular diagnostic test kits, during a government organized tour for journalists in Beijing, Thursday, May 14, 2020. China reported three new coronavirus cases Thursday while moving to reopen for business and schools. (AP Photo/Mark Schiefelbein)


One of the concerns often cited by those who fear opening up the economy too soon is the impact of asymptomatic carriers on the rest of the population. Are those who are completely unaware they’ve been infected with COVID-19 able to spread the disease?

Earlier this month, the National Institutes of Health (NIH) published the results of a study of 455 people, identified through contact tracing, who had some degree of contact with one asymptomatic carrier. (The full study can be viewed here.)

A 22-year-old woman with a medical history of congenital heart disease (CHD) had suffered from shortness of breath for 16 years. For a month, her shortness of breath had been worse than usual and she went to the emergency room of a Guangdong hospital on January 13. “The accompanied symptom was chest distress, without cough, sputum production and fever. Apart from CHD, she had no other diseases and had no smoking habit. Her temperature was normal, and laboratory measurements showed no apparent abnormalities. Echocardiography displayed atrial septal defect and severe pulmonary hypertension. The diagnosis was congenital heart disease, atrial septal defect and pulmonary hypertension.”

This young woman was treated and within three days, her condition had improved and she was transferred to the emergency department observation unit (EDOU). During a February 11 examination, she tested positive for COVID-19 and was immediately admitted to the quarantine ward in the infectious diseases department.


Notably, in isolation, she did not experience any symptoms typically associated with COVID-19.

She had no idea how or when she had become infected. By then, she had interacted with 455 individuals. Medical information about these contacts was gathered and a study was conducted.

They were divided into three groups: 35 patients, 196 family members and 224 hospital staffs. We extracted their epidemiological information, clinical records, auxiliary examination results and therapeutic schedules.

Results: The median contact time for patients was four days and that for family members was five days. Cardiovascular disease accounted for 25% among original diseases of patients. Apart from hospital staffs, both patients and family members were isolated medically. During the quarantine, seven patients plus one family member appeared new respiratory symptoms, where fever was the most common one. The blood counts in most contacts were within a normal range. All CT images showed no sign of COVID-19 infection. No severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections was detected in 455 contacts by nucleic acid test.

Conclusion: In summary, all the 455 contacts were excluded from SARS-CoV-2 infection and we conclude that the infectivity of some asymptomatic SARS-CoV-2 carriers might be weak.

This, of course, is good news.

On the other hand, in an earlier article, the NIH presents the case of an asymptomatic carrier whom they believe did transmit COVID to others. They cite a man who was released from a Chinese prison on February 25. He was told to self-quarantine for 14 days because a prison officer was confirmed to have COVID‐19 infection on February 14. He failed to do so and his brother, with whom he had spent time with early on became infected. The former prisoner, who had exhibited no symptoms of the virus, tested positive. Now, obviously the brother may have contracted COVID-19 from another source, but the extended periods of time the two spent together make it more likely he had picked it up from his asymptomatic brother.


The NIH also documents cases where those living with people who later learned they’d had COVID had not become infected as well as cases where one or more members of a household became infected while another did not.

So what does all of this prove? My unscientific take is that asymptomatic carriers are considerably less likely to pass on the disease than those who exhibit symptoms.

The NIH’s conclusion? “COVID‐19 transmission through asymptomatic carriers is a challenge to containment.” Decisive as always.



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