Patients’ viral loads are highest within five days of their first symptoms, emphasizing the need to isolate early to prevent transmission.

Since early March, a number of studies have suggested that patients infected with SARS-CoV-2 generally have a long incubation period and are most infectious early on in their sickness, with viral loads dropping off after nine days. These results have informed guidelines put forth by public health bodies, such as the US Centers for Disease Control and Prevention, which recommend that people isolate for at least 10 days following a positive COVID-19 test.

A meta-analysis published November 19 in The Lancet Microbe reviewed 98 studies on coronaviruses—79 of which focused on SARS-CoV-2 and the rest on SARS-CoV and MERS-CoV—to determine when patients’ viral loads are highest and could therefore transmit the virus most effectively. Of the SARS-CoV-2 studies, 73 included hospitalized patients only.

Their report finds that patients with COVID-19 show the highest amount of live virus in the upper respiratory tract within the first five days of showing symptoms, while the highest amount of virus in SARS and MERS is seen during the second week after showing symptoms. Despite finding high viral RNA loads, no study in the review isolated live virus beyond day nine after the first symptoms of COVID-19.

The Scientist spoke with Muge Cevik, a virologist and a clinician at the University of St Andrews and the lead author of the analysis, about the findings.

The Scientist: What inspired you to do this research?

Muge Cevik: Around April, we were writing a review paper mainly for clinicians. In writing that paper, we realized that we don’t have a good understanding about viral load dynamics and infectiousness periods not only for [SARS-CoV-2] but also for [SARS-CoV] and MERS. With my colleague from Glasgow University, we set up this systematic review and recruited four doctors to work together because systematic reviews are quite a huge task. At the beginning of the pandemic—I’m a clinician as well—we were not sure when to stop isolating patients, when we can safely discharge patients, and when patients stop being infectious. These have important implications for cases in the community but also cases in the hospital setting

TS: How did you go about deciding which studies you were going to include in your analysis?

MC: In our analysis, we included papers mainly looking at viral load dynamics. We wanted to include large studies—that’s the reason we excluded case reports or case series with less than five patients because generally [they] report atypical, unusual cases, and that’s typically those who basically shed virus for a long time.

We didn’t want our subjective opinion to influence the results, so specifically we included studies that counted the viral shedding from the time of symptom onset, not from time of hospitalization, and some studies actually included patients after discharge from hospital, so we didn’t know when the symptoms started. We tried to make it really similar across all studies, and that’s the reason our inclusion criteria were really strict.