Case Study Shows Asymptomatic Transmission of COVID-19 in China

March 30, 2020

Transmission of COVID-19 appears to be possible even among people who show no symptoms of the disease, according to a case study of a family cluster in China.

A case study of a family in Zhejiang province, China, shows how coronavirus disease 2019 (COVID-19) can spread before symptoms of the disease emerge, and symptoms can vary widely even in a close family cluster.

The study, published in Clinical Infectious Diseases, details how SARS-CoV-2 spread from a presymptomatic carrier to infect 8 of 9 family members from 3 households after a 58-year-old woman (index 1) and 60-year-old man (index 2) attended a Chinese Spring Festival temple activity on January 19.

Investigators at Ningbo First Hospital and the University of Nottingham Ningbo China in Zhejiang province examined case for the study, which was funded by the Natural Science Foundation of Zhejiang province.

Among the patients who contracted the disease, 2 remained asymptomatic and 1 was placed under intensive care with severe pneumonia.

The index couple’s daughter (case 1), son-in-law (case 2) and 2 grandchildren, ages 6 (case 3) and 13 months (case 4), visited the couple after the festival but before the emergence of symptoms, from January 20-23. On January 23, the young family (cases 1-4) visited the mother (case 5), father (case 6) and grandmother (case 7) of case 2.

Index 1 developed symptoms of headache, fatigue and fever on January 24, went to the hospital on January 26 and was diagnosed with COVID-19 on January 29. After the diagnosis, the patient’s husband and the 4 family members who had stayed with them were admitted to the hospital under observation. Her husband remained symptom-free but tested positive for the virus on February 1.

Case 1 began experiencing fever and cough on January 27, and case 2 developed a fever February 1.

The 6-year-old girl was the only family member who wasn’t diagnosed with SARS-CoV-2. The other child, a 13-month-old girl, tested positive for the virus on February 5 but remained symptom-free.

The third household of family members were admitted to the hospital for observation on February 1 and subsequently tested positive. Case 6, a 57-year-old man, was transferred to the intensive care unit on February 11 as his conditioned worsened.

The case study demonstrates that COVID-19 is clinically diverse, transmittable during the incubation period and can result in infected patients experiencing no symptoms. The study found that the viral loads were similar between symptomatic and asymptomatic patients.

“These ‘silent patients’ may remain undiagnosed and be able to spread the disease to large number of people,” the study noted.

“In summary, there are variations across individuals in the clinical manifestations of COVID19. From our reports, we should pay attention to how to prevent people from being infected by asymptomatic patients and patients who were in their incubation period.”

The incubation period for COVID-19 is estimated to be 5 days on average, with 97.5% of patients who develop symptoms doing so within 11.5 days of infections, a recent study showed.

The large number of patients infected with the virus who show few or no symptoms—estimated to be as high as 80% — complicates fighting the disease, forcing health care workers worldwide to shift their focus from containing the disease to mitigation, a recent report noted.

Helping providers in this effort, the National Institutes of Health has added a section to its website to provide training to those at risk of exposure to the virus on techniques, policies and procedures to help reduce the risk.