Coronavirus: Backwards tracing puts skids on superspreaders

Baroness Harding, head of NHS Test and Track, and her advisers have a “Light Bulb” moment. – Owl

Tom Whipple, Science Editor | Chris Smyth 

Using “backwards contact tracing” to identify the source of a virus cluster could double the effectiveness of the test-and-trace service, scientists say.

By working out where people got infected as well as who they went on to infect, it may be possible to stop twice as many infections being passed on.

There is increasing evidence that the virus transmits through “superspreading events”, according to a paper co-authored with researchers who sit on the government’s Sage committee.

Conventional tracing seeks to identify cases and track down the people they met after they became infectious. This is the method being employed in the UK. Backwards tracing, used in some Asian countries, seeks to identify the person who infected that first case.

Baroness Harding of Winscombe, head of NHS Test and Trace, recently had a version tested in Leicester. She promised to begin backwards contact tracing but has not committed to a date amid concerns that it would prove too labour-intensive.

In the UK the R number is about 1. This means that on average each person infects one other. However, this is only an average. Most people infect no one while a few people infect many others. This, scientists say, makes the disease especially suitable for backwards contact tracing.

“We estimate that probably about 80 per cent of transmission comes from about 10 per cent of cases,” Adam Kucharski, of the London School of Hygiene and Tropical Medicine, said.

This is a headache for public health officials because clusters can appear suddenly as a result of “superspreading events”. It is also an opportunity.

“If you have superspreading, if you identify a case, the chances are it will have come from an event or cluster that’s generated a lot of other cases too,” Dr Kucharski said. “If you’re just asking who those people infected you might get quite a few cases but if you’re asking where did they get infected that will often lead you to a sizable cluster.

“What a number of countries are doing — Japan and South Korea, for example — is really working very hard to identify people who are associated with large transmission events that can give you a disproportionate effect.”

Dr Kucharski has co-authored a paper with colleagues at the London School of Hygiene and Tropical Medicine, including Graham Medley, who sits on the Sage committee. It estimates that if the test-and-trace system were able to include backwards tracing there could be dramatic benefits. “Backward tracing increases this maximum number of traceable individuals by a factor of two to three,” they write in a paper published online before peer review.

At the moment, contact tracers only ask infected people about their movements on the two days before they got ill. Backwards contact tracing means asking them about the previous 14 days, taking up much more of tracers’ time. It had been hoped that infections could be driven down before beginning this strategy. In Leicester, the method is credited with helping to control infection rates, but has been given less importance nationally.

NHS Test and Trace is expected to launch a scaled-back version of its troubled coronavirus app today. The Times revealed last week that after the failure of trials of automated contact tracing the app has been changed to a source of personalised information, including infection levels in local areas.

Officials insist that the app will still contain some element of contact tracing, such as telling people that they have been near someone who later tested positive. But the Bluetooth signalling which shows that phones have been near each other is not yet trusted on its own as a basis for instructing people to isolate for two weeks. Instead Test and Trace will continue to rely on human contact tracers to decide.

However, Professor Christophe Fraser, a scientific adviser to the Department of Health, told the BBC: “We need the app to help stop transmission by tracing close-proximity contacts as quickly and as comprehensively as possible, capturing those contacts we don’t know or don’t remember meeting.

“The app should enable us to return to more normal daily activities with the reassurance that our contacts can be rapidly and anonymously notified if we get infected.”