Covid-19 spread by presymptomatic carriers

Author: Mark Gould

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An analysis* of Covid-19 infections in Singapore and China has revealed that large numbers of people appear to have been infected by symptom-free carriers who were incubating the virus.

In the Singapore cluster, between 45% and 84% of infections appeared to come from people incubating the virus. In Tianjin in China, the figures ranged from 65% to as much as 87% according to an emerging medical research, which has yet to be peer-reviewed and should not be used to guide clinical practice.


However the findings are a worry for infectious disease researchers as it means that isolating people once they start to feel ill will be far less effective at slowing the pandemic than had been hoped.

“This is one of the first things we were worried about when the outbreak began,” said Steven Riley, professor of infectious disease dynamics at Imperial College London, who was not involved in the work. “It was certainly unhoped for. This is one thing we really didn’t want to go this way,” he told The Guardian.

Researchers in Belgium and the Netherlands drew on data from outbreaks in Singapore and Tianjin to work out the “generation interval” for Covid-19. The generation interval is the time between one person getting infected and them infecting another. The figure is valuable for estimating the speed at which an outbreak will unfold.

The mean generation interval was 5.2 days in the Singapore cluster and 3.95 days in the China cluster. The scientists went on to calculate what proportion of infections were likely spread from people who were still incubating the virus and had yet to develop symptoms.

The researchers urge some caution over their findings as they did not have precise information on who infected whom in the two clusters of disease. But even the lowest estimates show there was substantial transmission of coronavirus from people who had yet to fall ill.

Tapiwa Ganyani, a researcher on the team, said the numbers suggest that isolating sick people would not be enough to quell the outbreak. “It is unlikely that these measures alone will suffice to control the Covid-19 epidemic,” he said. “Additional measures, such as social distancing, are required.”

The finding confirms recent comments from the head of the World Health Organisation’s emerging diseases and zoonoses unit, who said preliminary data showed patients shed more virus in the early stages of the disease, including when presymptomatic.

A separate study** that tracked nine patients in Germany found that levels of the virus might already be in decline when symptoms come on.

“The number helps us estimate how likely certain kinds of public health interventions are to work,” said Riley. “If you ask people to go home when they feel ill and they have infected half the people they are going to infect before they feel ill, then you know you are not going to get rid of those transmissions.” But he said it was still important to isolate those with symptoms, even if it would not be enough to stop the outbreak.


*Ganyani T, Kremer C, Chen D, et al. Estimating the generation interval for COVID-19 based on symptom onset data. medRxiv, 8 March 2020. DOI: 10.1101/2020.03.05.20031815

**Woelfel R, Corman M, Guggemos W, et al. Clinical presentation and virological assessment of hospitalized cases of coronavirus disease 2019 in a travel-associated transmission cluster. medRxiv, 12 March 2020. DOI:10.1101/2020.03.05.20030502

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