Most virus transmission ‘comes before signs appear’
TEST and trace systems are missing Covid cases because much of transmission occurs in the five days before symptoms show, research suggests.
The study by Oxford University shows that the peak of transmission occurred just before and after the onset of symptoms. However, 42 per cent of cases were spread in the five days before any signs of the virus appear.
Currently, the NHS test and trace system only asks people who test positive for Covid to provide details of those with whom they have been in contact in the 48 hours before testing. But scientists said this system would miss significant numbers of those who are likely to have caught the virus.
Dr Luca Ferretti, lead author and senior researcher at Oxford University’s Nuffield Department of Medicine, said: “The large fraction of transmissions that occur either before or shortly after onset of symptoms means that isolation of cases starting more than two days after onset of symptoms is far from sufficient to control the epidemic.”
Researchers suggested that limiting contact tracing to two days before symptoms show, rather than five, is likely to miss around 16 per cent of onward transmissions.
The estimates came from modelling, based on four papers which 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 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 Covid 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 showed.
He said those who developed mild or non-specific symptoms, which might mean that they have the virus, should be particularly strict about adhering to social distancing, handwashing, maskwearing 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 though 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 was 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.