Asymptomatic COVID-19 transmission occurs in more than 1 in 10 cases

It takes just less than 4 days for COVID-19 to spread from one person to another and cause symptoms, and more than 10 percent of the cases are infected by a person who has caught the virus but yet to show symptoms, recent studies suggest.

“Asymptomatic transmission definitely makes containment more difficult,” said study principal investigator Professor Lauren Ancel Meyers of the University of Texas at Austin, Austin, Texas, US.

“This tells us that COVID-19 outbreaks can be elusive and require extreme measures.”

Looking into 468 COVID-19 transmission events that were reported in China outside of Hubei province, the investigators found that the time it took for symptoms to show between two successive cases within a transmission chain — ie, the serial interval — was 3.96 days (95 percent confidence interval [CI], 3.53–4.39 days). [Emerg Infect Dis 2020;doi:10.3201/eid2606.200357]

This, they noted, was much shorter than those reported for SARS* at a mean of 8.4 days and for MERS** at 14.6 days.

“Ebola, with a serial interval of several weeks, is much easier to contain than influenza, with a serial interval of only a few days. Public health responders to Ebola outbreaks have much more time to identify and isolate cases before they infect others,” said Meyers.

“The data suggest that this coronavirus may spread like the flu. That means we need to move quickly and aggressively to curb the emerging threat,” she added.

In addition, the serial interval was slightly shorter for transmission occurring within the household compared with that outside the household (mean, 4.03 vs 4.56 days), although the difference was not significant.

Of concern, 12.6 percent of the cases had symptoms even before their source infectors showed any symptoms.

Stealth mode

The finding that presymptomatic transmission does occur was supported by a new study on 243 cases in Singapore. Analysis of seven COVID-19 clusters revealed that 10 out of 157 locally acquired cases (6.7 percent) were attributed to presymptomatic transmission. [MMWR Morb Mortal Wkly Rep 2020;69:1-5]

In four of the seven clusters, presymptomatic transmission exposures have likely occurred 1–3 days before symptoms appeared in the source infectors. 

“The possibility of presymptomatic transmission increases the challenges of containment measures. Public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of presymptomatic transmission,” suggested the researchers led by Dr Vernon Lee from Ministry of Health, Singapore.

“The potential for presymptomatic transmission underscores the importance of social distancing, including the avoidance of congregate settings, to reduce COVID-19 spread,” they stressed.

In fact, the possibility of silent transmission by infected persons who were asymptomatic has been raised earlier in another study on 28 COVID-19 patients in Japan, which showed that the median serial interval was 4.0 days. [Int J Infect Dis 2020;93:284-286]

“[The duration] is close to or shorter than its median incubation period … [which] suggests that a substantial proportion of secondary transmission may occur prior to illness onset,” the researchers noted. 

According to Meyers, the findings that the virus can spread quickly and silently hold implications for public health, and could provide guidance on how to contain the outbreak.

“[Our study] provides evidence that extensive control measures including isolation, quarantine, school closures, travel restrictions, and cancellation of mass gatherings may be warranted,” she said.