Dr. Kerkhove of WHO Comments on Asymptomatic COVID-19 Cases

Updated June 9, 11 AM

The World Health Organization’s (WHO) Maria Van Kerkhove, PhD, said in a press briefing Monday evening that asymptomatic individuals are less likely to transmit SARS-CoV-2 than individuals with symptoms.

The statement made headlines, though it was already theorized that pre-symptomatic transmission might be more critical to spread than asymptomatic transmission. Individuals might also self-report an asymptomatic case if they have mild symptoms.

Kerkhove also acknowledged that asymptomatic and presymptomatic spread have been staples of past studies modeling transmission dynamics in nursing homes and household settings. She cited a "number of countries" reporting detailed contact tracing results, showing secondary transmission is a very rare circumstance. Much of this data, she added, has not been published.

She stressed the importance that countries conducting contact tracing provide more data to help "truly answer this question."

"It still appears to be rare that an symptomatic individual transmits onward," Kerkhove said. "When we go actually go back and say how many them were truly asymptomatic, we find out that many have really mild disease."

A recent study on SARS-CoV-2 testing in a long term care facility found that over half of residents with positive test results were asymptomatic at the time of testing. What role these residents played in spread isn't certain, and there may be factors such as viral load that make asymptomatic cases more or less important in spread based on setting.

In another recent study, based on contact tracing in Taiwan, investigators reviewed 2761 close contacts of 100 SARS-CoV-2 positive patients (5.5% were household contacts, 2.8% were non-household family contacts, and 25.3% were health care contacts). Ultimately, 22 secondary cases were identified and interestingly, none of the secondary cases were related to the 9 asymptomatic index cases.

1/2@WHO recently published a summary of transmission of #COVID19, incl. symptomatic, pre-symptomatic and asymptomatic transmission

See page 2 of👇https://t.co/2OJ2pLT5Iu

In this summary, we state: "Comprehensive studies...

— Maria Van Kerkhove (@mvankerkhove) June 8, 2020

2/2 ... on transmission from asymptomatic individuals are difficult to conduct, but the available evidence from contact tracing reported by Member States suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms

— Maria Van Kerkhove (@mvankerkhove) June 8, 2020