COVID-19 Resource Center

COVID-19 Resource Center

Perspective from Amesh A. Adalja, MD
Disclosures: The authors report no relevant financial disclosures.
November 30, 2020
3 min read

SARS-CoV-2 infectiousness peaks in first week after symptom onset

Perspective from Amesh A. Adalja, MD
Disclosures: The authors report no relevant financial disclosures.
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SARS-CoV-2 infectiousness appears to peak in the upper respiratory tract in the first week of illness — earlier than other coronaviruses, which may explain the rapid spread of COVID-19, according to researchers.

Muge Cevik, MD, MSc, MRCP, a clinical lecturer in infectious diseases and medical virology at the University of St. Andrews in Scotland, and colleagues conducted a systematic review and meta-analysis of 98 studies on SARS-CoV, SARS-CoV-2 and MERS-CoV.

The review included 79 studies on SARS-CoV-2 consisting of 5,340 individuals, eight studies on SARS-CoV consisting of 1,858 individuals, and 11 studies on MERS-CoV that included 799 individuals.

Of the 79 studies focusing on SARS-CoV-2, 43 of them reported on the duration of shedding in the upper respiratory tract, showing a mean viral shedding duration of 17 days (95% CI, 15.5-18.6). Seven studies reported on the shedding in the lower respiratory tract, with a mean of 14.6 days (95% CI, 9.3-20). In 13 studies, the mean duration of viral shedding in stool samples was 17.2 days (95% CI, 14.4-20.1), and in two studies the mean duration of viral shedding in serum samples was 16.6 days (95% CI, 3.6-29.7).

Eight studies that evaluated SARS-CoV-2 viral load in serial upper respiratory tract samples showed peak viral loads within the first week of symptom onset, the researchers reported. The highest viral loads were reported at the time of symptom onset or soon after, at around days 3 to 5 of illness, according to Cevik and colleagues. In the five studies evaluating viral loads in lower respiratory tract samples, the researchers observed that a peak occurred in the second week of illness.

“Our findings are in line with contact tracing studies, which suggest the majority of viral transmission events occur very early, and especially within the first 5 days after symptom onset, indicating the importance of self-isolation immediately after symptoms start,” Cevik said in a news release.

In the eight studies on SARS-CoV, the maximum duration of viral shedding reported was after 8 weeks in the upper respiratory tract, 52 days in the lower respiratory tract, 6 to 7 weeks in serum and 126 days in stool samples, the researchers reported. The analysis found low viral loads in the initial days of the illness, which increased after the first week and peaked between days 10 and 14 in the upper respiratory tract.

Mean shedding duration for MERS-CoV came at 15.3 days in the upper respiratory tract (95% CI, 11.6-19), and 16.3 days in the lower respiratory tract (95% CI, 13.8-18.9), Cevik and colleagues reported. Only one study reported on the duration of viral shedding in serum, which came at a maximum of 34 days. Peak viral loads were observed between 7 and 10 days, the researchers noted.

Cevik said public awareness needs to be raised in regard to the range of symptoms related to COVID-19, “including mild symptoms that may occur earlier on in the course of the infection than those that are more prominent, like cough or fever.”

Twelve studies reported on asymptomatic individuals with SARS-CoV-2, and six reported on how quickly these individuals cleared the virus.

“Although viral RNA loads appear to be largely similar between those with and without symptoms, a few studies suggest that asymptomatic individuals might clear the viral material from their bodies faster,” Cevik said. “Several studies have found that individuals with asymptomatic infection may clear the virus faster, suggesting that those without symptoms may be as infectious as those with symptoms at the beginning of infection but may be infectious for a shorter period.”

Cevik noted that there are limited data available on the shedding of virus in asymptomatic individuals; thus, any policy change on quarantine duration in the absence of testing would be premature.

“This is the first systematic review and meta-analysis that has comprehensively examined and compared viral load and shedding for these three human coronaviruses,” Cevik said. “It provides a clear explanation for why SARS-CoV-2 spreads more efficiently than SARS-CoV and MERS-CoV and is so much more difficult to contain.”