London School of Hygiene & Tropical Medicine

Backward contact tracing combined with standard contact tracing has the potential to double the effectiveness of the NHS Test and Trace system (NHSTT), according to new preliminary research by the London School of Hygiene & Tropical Medicine.

Previous modelling has suggested that tracing non-household contacts could potentially reduce transmission by an additional 5–15% on top of self-isolation and household quarantine.

This study, not yet peer reviewed, estimates that adding backward contact tracing to the system could make it 2-3 times more effective.

Backward tracing is an outbreak investigation process where new cases and their contacts are routinely linked via the events or place where they originally got infected. Other infections linked to these events also have their contacts traced. This is opposed to standard ‘forward’ contact tracing which looks forward, and quarantines contacts of new cases to prevent future transmission.

NHS Test and Trace system now employs both backward and forward contact tracing.

Backward tracing is good at finding large “group transmissions” because even if some cases are missed (for example, due to asymptomatic infections), others still could point to the shared source of exposure.

Based on current UK dynamics with suppressed transmission and frequent superspreading events observed worldwide, the research team from LSHTM’s Centre for the Mathematical Modelling of Infectious Diseases, estimate at least one in four backward tracing practices could identify groups of five or more COVID infected people.

Previous work by LSHTM and others has shown that ‘superspreading events’ are important for COVID-19 to spread. A small proportion of infections are typically behind a large proportion of transmission. This means that cases tend to occur in clusters, with one case generating several secondary infections.

Akira Endo, a member of the research team, said: “Effective contact tracing will enable us to live our lives as normally as possible during the COVID-19 pandemic. COVID-19 thrives on close contact between large groups of people. If we can trace back to these ‘hotspots’ we could potentially flag multiple other infections.

“We’ve shown that combining this with backwards contact tracing could make a big difference in stopping further transmission, if implemented quickly and at scale. This approach, which has been taken in South Korea and Japan, relies upon good co-ordination between epidemiological, testing and quarantine operations to be successful.”

There are some challenges to achieving effective backward tracing. One of the biggest issues is delays in tracing. This study assumes that contacts of backward-traced cases (that is, “next generation” of those linked to the identified shared source of infection) are reached and quarantined before they become infectious.

Given the typical time interval of 4-6 days between infection generations, contact tracing needs to be implemented at a similar time scale to be most successful; otherwise, some identified contacts may have already infected others when they start to quarantine.

Publication

**This study has not yet been peer-reviewed**

Akira Endo, Quentin Leclerc, Gwen Knight, Graham Medley, Katherine E Atkins, Sebastian Funk & Adam J Kucharski. Implication of backward contact tracing in the presence of overdispersed transmission in COVID-19 outbreaks. DOI: 10.1101/2020.08.01.20166595