Patients spread coronavirus two to three days before symptoms appear: Study

Patients with COVID-19 shed, or excrete, infectious coronavirus two to three days before showing first symptoms, according to a modelling study which says control measures taken after symptoms appear may have a reduced effect in curbing the pandemic.

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The study, published in the journal Nature Medicine, noted that several factors can affect the efficacy of control measures designed to prevent the spread of the novel coronavirus, SARS-CoV-2.

These include the time between successive cases in a chain of transmission, and the incubation period between exposure to an infection and the appearance of symptoms, according to researchers, including Eric Lau from the University of Hong Kong.

If the serial interval is shorter than the incubation period, this would indicate that transmission may have happened before overt symptoms develop, they said.

In the study, the scientists assessed the temporal patterns of viral shedding in 94 patients with COVID-19 admitted to Guangzhou Eighth People’s Hospital in China.

They analysed throat swabs collected from these patients from when symptoms first appeared until 32 days later.

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According to the study 414 swabs were analysed, and the scientists found that the patients had the highest viral load at the onset of symptoms.

They also modelled COVID-19 infectiousness profiles from a different sample of 77 ‘transmission pairs’ from publicly available data.

Each of these pairs comprised two patients with COVID-19 with a clear epidemiological link, with one patient highly likely to have infected the other, the scientists explained.

From here, the researchers inferred that infectiousness started 2.3 days before symptoms appeared and peaked at 0.7 days before their appearance.

By their estimate 44 per cent of secondary cases were infected during the pre-symptomatic stage, with infectiousness predicted to decrease quickly within seven days.

According to the scientists, limitations of the study include the reliance of the research on patient recall of the onset of symptoms, which may have introduced bias, as there may be a delay in the recognition of the first symptoms.