SARS-CoV-2 shedding prolonged, but viable viruses short-lived

Shedding of SARS-CoV-2 RNA can persist for a long time, particularly in stool and respiratory samples, according to a recent systematic review and meta-analysis. However, viable viruses are relatively short-lived and are highly unlikely to be detected 2 weeks after illness onset.

Moreover, “[m]ost studies detected the SARS-CoV-2 viral load peak within the first week of illness,” the researchers said. “These findings highlight that isolation practices should be commenced with the start of first symptoms, including mild and atypical symptoms that precede more typical novel coronavirus disease (COVID-19) symptoms.”

Seventy-nine studies (n=5,340) related to shedding of SARS-CoV-2 RNA and viable virus, as well as to its viral load dynamics, were included. Case series with at least five patients, cohort studies, and randomized controlled trials were eligible for inclusion, as were literature from preprint servers. At the same time, eight (n=1,858) and 11 (n=799) studies on SARS-CoV and MERS-CoV were also included.

Forty-three studies found that viral RNA shedding in the upper respiratory tract could last for an average of 17.0 days; 13 studies reported a mean shedding duration of 17.2 days in stool samples. Notably, the maximum longest reported duration of RNA shedding was 83 days in the upper respiratory tract and 126 days in stool. [Lancet Microbe 2021;2:e13-e22]

Eleven studies tried to isolate viable viruses, eight of which used respiratory samples. All attempts to culture the virus were successful within the first week of illness. No viable viruses could be obtained after day 9 of symptoms despite the persistence of high viral RNA levels.

Viral isolation seemed to depend on viral load. One study could not obtain successful cultures at loads below 1×10⁶ copies per mL, while another found a cycle threshold cutoff of 24. Culture positivity declined with increasing cycle thresholds values.

In terms of viral load dynamics, eight studies used serial upper respiratory tract samples for quantification. Peak viral levels occurred within the first week of symptom onset, around day 3–5 of illness. Titres dropped consistently thereafter. In the lower respiratory tract, viral load peaked by the second week of illness.

Notably, load trends were erratic in stool samples. Some studies reported peak levels on day 7 of illness or 2–3 weeks after. Others detected the highest viral stool titres 5–6 weeks after symptom onset.

Compared against other human coronaviruses, SARS-CoV-2 showed distinct patterns of viral shedding and persistence. In SARS-CoV, for instance, shedding lasted up for only 8 weeks in the upper respiratory tract and 126 days in the stool. Viral load started out low during the initial days, ramping up and peaking at days 10–14 in upper respiratory samples; declines were observed after weeks 3–4.

MERS-CoV, in comparison, had a shedding duration of 15.3 days in the upper respiratory tract, and peak viral loads were between days 7 and 10.

“To our knowledge, this is the first systematic review to comprehensively examine and compare SARS-CoV-2, SARS-CoV, and MERS-CoV viral dynamics, and the first meta-analysis of viral shedding duration,” the researchers said, adding that their findings “provide detailed understanding about the evidence available so far on viral dynamics of SARS-CoV-2 and has implications for pandemic control strategies and infection control practices.”