Test and trace systems are missing Covid-19 cases because much of transmission occurs in the five days before symptoms show, research suggests. A 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 appears. Currently, the NHS Test and Trace system only asks people who test positive for Covid-19 to provide details of those with whom they have been in contact with in the 48 hours before testing. But scientists said this system would miss significant numbers of those likely to have caught the virus. Lead author Dr Luca Ferretti, 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 (see graphic below) show, rather than five, is likely to miss around 16 per cent of onward transmissions.
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