A team of scientists from the UK conducted a systematic overview 98 studies that analyzed the change in viral load of patients with SARS-Cov-2, SARS and MERS. The results are published in The Lancet Microbe.
The analysis of the studies showed that the average duration of the isolation of RNA of the coronavirus was 17 days from the onset of symptoms in the upper respiratory tract, 14.6 days – in the lower respiratory tract, in feces – 17.2 days. In one case, the virus was isolated within 83 days. However, none of the studies found live viruses in tissues or secretions 9 days after the onset of symptoms. Thus, the peak of the viral load occurs in the first week of the disease, scientists say. In SARS patients, live virus shedding peaked on days 10-14, with MERS – on days 7-10.
The researchers said the findings are consistent with numerous contact tracing studies showing that most transmission occurs in the early days after symptoms appear. Therefore, the authors considered the recommendations to isolate contact people for 10 days justified.
The article emphasizes that the systematic review included studies that tracked the viral load of only hospitalized patients, that is, with moderate to severe COVID-19. It is possible that people with mild disease and asymptomatic carriers have a shorter shedding period for the live virus.
The question of when and how much a person with COVID-19 is contagious remains one of the most pressing, since the effectiveness of measures to reduce the incidence depends on it. So, in October, the Centers for Disease Control and Prevention (CDC) expanded data on the risks of infection and recommendations for isolating people with confirmed COVID-19. According to American doctors, the concentration of RNA coronavirus in the upper respiratory tract is guaranteed to decrease after the onset of symptoms. This means that a person is most contagious before they develop symptoms, not after.
Virus replication is significantly reduced by the tenth day from the onset of symptoms. In patients with severe COVID-19, the virus can multiply for up to 20 days, but in 95% of cases, this process stops after 15 days. In mild to moderate cases, the CDC recommends self-isolation for at least 10 days after the onset of symptoms, in severe cases, 20 days.