Asymptomatic COVID-19 cases carry as much virus as those with symptoms

Up to one-fifth of individuals with COVID-19 are asymptomatic, with many of them harbouring as much coronavirus as patients who are symptomatic, a study shows — raising concerns that asymptomatic carriers may drive community spread of COVID-19.

“Considering that most asymptomatic individuals with COVID-19 are likely to go unnoticed by healthcare workers and continue to reside within communities, such individuals may act as an essential driving force for the community spread of COVID-19 and the ongoing pandemic state,” the researchers pointed out. 

“Our data add further support to the general public use of face masks, regardless of the presence of symptoms, and suggest that the scope of SARS-CoV-2 testing should be expanded to include asymptomatic individuals in high-risk settings, such as nursing homes or healthcare facilities,” said principal investigator Professor Sung-Han Kim from Asan Medical Center, Seoul, Korea, in an accompanying podcast.

Among the 213 patients with COVID-19 recruited in a community isolation facility in South Korea, as high as 19 percent had remained asymptomatic during the observation period from exposure to admission. [Thorax 2020;doi:10.1136/thoraxjnl-2020-215042]

After a median of 13 days, 95 percent (n=39) of the asymptomatic participants and 84 percent (n=144) of the symptomatic patients subsequently underwent a follow-up testing by RT-PCR.

In the follow-up PCR testing, 54 percent of the asymptomatic participants and 64 percent of the symptomatic patients continued to test positive.

“These results suggest that patients with COVID-19 may experience a more protracted course than that initially hypothesized,” said Kim and co-authors.

“Indeed, we found that asymptomatic individuals had RT-PCR Ct values for SARS-CoV-2 genes comparable with those of patients with mild symptoms of COVID-19,” they noted.

The mean Ct values for the E (31.15 vs 31.43; p>0.99), RdRp (32.26 vs 32.93; p=0.92) and N (33.05 vs 33.28; p>0.99) genes of SARS-CoV-2 did not differ significantly between the asymptomatic and the symptomatic patients — indicating that asymptomatic individuals harboured similar viral loads as those who were symptomatic.

“However, our results should be interpreted with caution because positive RT-PCR results do not necessarily indicate the presence of viable virus, for which cell culture is needed for confirmation,” the researchers cautioned.

They suggested further studies to evaluate the transmissibility of SARS-CoV-2 from individuals who are asymptomatic, in order to inform recommendation on a continuous need for precaution (such as universal personal protective equipment) and quarantine.

“Until such time as further data become available regarding the duration and transmissibility of viable virus shedding from asymptomatic individuals, [SARS-CoV-2 testing should be extended to certain groups as a precautionary measure],” suggested Kim and co-authors.

Why some people are asymptomatic

Another study showed that alveolar macrophages were unable to mount an interferon (IFN) response when challenged with SARS‐CoV‐2 in vitro, in contrast to influenza A virus which triggered a robust antiviral response. [EMBO Rep 2020;doi:10.15252/embr.202051252]

The findings suggest that SARS‐CoV‐2 was able to conceal its genetic material from being recognised by alveolar macrophages and thereby evade immune detection — shedding light into how some people can get infected with SARS-CoV-2 and yet, remain asymptomatic, especially during the early stages of infection.