Asymptomatic transmission may account for half of all COVID-19 cases

SARS-CoV-2 transmission from asymptomatic individuals could account for more than half of all transmissions, according to estimates from a modelling study and data from a real-world study — suggesting that isolation of symptomatic cases alone is insufficient for controlling the ongoing spread of SARS-CoV-2.

“In the absence of effective and widespread use of therapeutics or vaccines that can shorten or eliminate infectivity, successful control of SARS-CoV-2 cannot rely solely on identifying and isolating symptomatic cases,” the researchers highlighted. “Even if implemented effectively, this strategy would be insufficient.”

While several vaccines have been rolled out to date, whether they will turn out as effective in preventing symptomatic COVID-19 illness in the real world remains to be seen, much less on preventing asymptomatic transmission.

The importance of asymptomatic transmission as a source of spread within the community has been suggested by epidemiological evidence and is further reinforced by the current decision analytical modelling study. [JAMA Netw Open 2021;doi:10.1001/jamanetworkopen.2020.35057]

In the study, the proportion of SARS-CoV-2 transmissions in the community that arose from asymptomatic individuals were compared with those from symptomatic cases using baseline assumptions that median incubation period was 5 days and infectious period lasted for 10 days, based on data from a previous meta-analysis. Asymptomatic individuals, by definition, included presymptomatic individuals (ie, infectious before developing symptoms) and those who never showed symptoms.  

The researchers found that an estimated 59 percent of all transmissions likely came from asymptomatic individuals — comprising 35 percent from presymptomatic persons and 24 percent from those who never showed any symptoms.

Varying the baseline assumptions on the proportion of transmission from those who were never symptomatic and their relative infectiousness revealed that asymptomatic transmission contributed to at least 50 percent, and up to 62 percent, of all transmissions.

“[These results] show that across broad ranges of possible assumptions, the finding that asymptomatic transmission is a critical component of SARS-CoV-2 transmission dynamics remains constant,” said the researchers.

“[Importantly,] these findings suggest that effective control also requires reducing the risk of transmission from people with infection who do not have symptoms,” they pointed out.

Until effective treatments and widespread adoption of effective vaccines are in place, continued containment measures including mask wearing and social distancing are warranted to reduce the risk of asymptomatic carriers from unknowingly spreading the virus.  

Real-world data in Singapore 

The role of asymptomatic transmission in the ongoing spread of COVID-19 is beyond doubt, with real-world evidence also showing that more than half of transmissions came from asymptomatic individuals.

Analysing 3,790 close contacts of 628 index cases in Singapore, 89 contacts turned out to have been infected. Among these, 56 percent of the transmissions originated from an asymptomatic index case — a proportion that is similar to the estimate from the modelling study above. [Lancet 2020;doi:10.1016/S0140-6736(20)32651-9] 

Nonetheless, the study suggests that even though asymptomatic individuals are infectious, they might be less infectious than those who were symptomatic.

“The incidence of COVID-19 among close contacts of a symptomatic index case was 3.85 times higher than for close contacts of an asymptomatic index case [p<0.0001],” reported the researchers led by Dr Andrew Sayampanathan from Ministry of Health, Singapore. 

Echoing the message from the above modelling study, Sayampanathan and team alluded that containment strategies should not be focused on isolating symptomatic cases alone.

“[Overall,], these findings suggest that where resources permit, contact tracing should proactively seek people with asymptomatic COVID-19 because they can transmit disease and will need to be contained if a national policy objective is to minimize cases and transmission,” said the researchers.

“However, if resources are limited, then focusing contact tracing around symptomatic people who are easy to identify … might be more resource-effective in reducing transmission at the population level,” they added.