The Guardian reports:

Many coronavirus infections may be spread by people who have recently caught the virus and have not yet begun to show symptoms, scientists have found.

An analysis of infections in Singapore and Tianjin in China revealed that two-thirds and three-quarters of people respectively appear to have caught it from others who were incubating the virus but still symptom-free.

The finding has dismayed 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.”

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, according to the analysis which is under review at an infectious disease journal. 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.

There are uncertainties in the figures because the scientists 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.

In the Singapore cluster, between 45% and 84% of infections appeared to come from people incubating the virus. In China, the figures ranged from 65% to as much as 87%. [Continue reading…]