Test and trace missing Covid cases because virus can be transmitted five days before symptoms show

The estimates came from modelling, based on four papers which have examined timings of transmission and onset of symptoms. The research concluded that around 35 per cent of all transmissions occurred on the day symptoms emerged, or on the following day.

The research only examined transmission timing in individuals who eventually developed symptoms. 

Other studies suggest that up to 80 per cent of those who become infected with coronavirus do not show symptoms. However, those who never develop symptoms are likely to be less infectious.

Dr Ferretti said the findings also showed the importance of physical distancing and mask-wearing, given the level of transmission occurring before signs of Covid-19 showed.

He said those who developed mild or non-specific symptoms should be particularly strict about adhering to social distancing, hand-washing, mask-wearing and reducing their contact with others in the first two to three days after signs appear.

Dr Ferretti added: “Public health authorities should consider appropriately longer pre-symptomatic intervals for contact tracing whenever an estimate of the date of infection is available and precedes symptoms by more than four to five days.”

Researchers said that although there was little evidence of transmission from five days after onset of symptoms, individuals should not consider they were “safe” at this point.

They said evidence of low transmission rates at this point is likely to be skewed by the fact that high numbers of people in this situation are already isolating from others, as well as due to lower biological infectiousness.

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