Asymptomatic individuals shown to transmit SARS-CoV-2 infection in Vietnam

Startling findings from an observational study conducted by OUCRU’s Dr Le Van Tan in collaboration with the Hospital for Tropical Diseases, Ho Chi Minh City CDC, the Department of Health and the OUCRU COVID-19 Research Group have shown that it is common for people who are infected with the virus that causes COVID-19 (SARS-CoV-2) to have no symptoms whatsoever.  By testing quarantined people in Vietnam, the team were able to detect the asymptomatic individuals. The virus disappeared faster from the bodies of the asymptomatic carriers than from that of symptomatic individuals, but it appeared that some of them still managed to pass the infection on to others.

Dr Tan and his team conducted a prospective study at a quarantine center for COVID-19 in Ho Chi Minh City, Vietnam. They enrolled quarantined people with confirmed SARS-CoV-2 infection, collecting clinical data, travel and contact history, and saliva at enrolment and daily nose and throat swabs for testing. The team then studied the natural history and transmission potential of asymptomatic and symptomatic individuals.

Between March 10th and April 4th, 2020, 14,000 quarantined people were tested, and 49 were positive. Of these, 30 participated in the study: 13(43%) never had symptoms.  Compared with symptomatic individuals, the people who had no symptoms were less likely to have detectable levels of the virus in their nose and throat swab samples. Only eight out of the 13 asymptomatic individuals had detectable levels of the virus in the samples when they first enrolled in the study, whereas 100% of the patients who were showing symptoms had detectable levels of the virus in their samples.

The research team also tested saliva samples from 27 the enrolled individuals.  RNA – small genetic material from the virus – was detected in 74% of all of the saliva samples; in 64% of the asymptomatic group and 81% of the symptomatic group. The detectability of the virus in saliva samples is good news for future development of rapid tests – since saliva is much easier to collect.  But the high levels of detectable virus in the asymptomatic patients is of concern, since it indicates that even asymptomatic individuals are likely to be able to spread the virus.

The team were able to show that the virus cleared faster from the individuals who had no symptoms – particularly in the first week of followup.  However, even though the virus cleared faster from the asymptomatic patients, and was less frequently detected in the samples taken from the patients, two of  the asymptomatic individuals appeared to transmit the infection to up to four contacts.

The researchers looked at the way the virus was transmitted in a community cluster in Ho Chi Minh City in Vietnam.  With very careful contact tracing and quarantine of contacts, it was possible to analyse how the patients came into contact with the virus.  In the cluster of seven patients who were enrolled in the study, two of the asymptomatic patients appeared to have transmitted the virus to their personal contacts.  The pathway of these transmissions is demonstrated in the figure below.

Dr Le Van Tan said: “Because we don’t know whether the people with whom we are in contact carry the virus or not, it is vital to maintain social distancing and wear a facemask in public. The results strongly support the ongoing rigorous containment approach applied in Vietnam.”

The World Economy Forum cited this publication as a first of the top science stories about COVID-19 in the week of 8-14 June 2020.  Data from Vietnam suggests asymptomatic patients can still transmit the virus to others.

You can download the full manuscript outlining the study and its findings from Clinical Infectious Diseases, here.