Backward Tracing

Not everybody with coronavirus is infectious to the same extent. It has been shown that many infected individuals infect just a few people, while a few infected individuals can end up infecting comparatively large numbers. Seats of infection can form around these small numbers of people. The aim of backward contact tracing is to find these seats of infection and stop them from spreading.

What is backward contact tracing?

Graph database displayed in contact tracing software. Blue represents the locations / events and red represents the index cases.

From a statistical point of view, it is highly likely that one person in a seat of infection has become infected, so the backward contact tracer goes back 14 days with the person concerned and records all of the events relevant to backward contact tracing within the period specified.

The relevant events / locations are summarised under the terms “CROWDED ENCLOSED CLOSE” (explained in detail below).


CROWDED: Locations where many people are present, e.g.   

  • Bars
  • Restaurants
  • Birthday parties / weddings

ENCLOSED: Enclosed and / or poorly ventilated spaces, e.g.

  • Fitness centres
  • Cultural events
  • Meetings

CLOSE: Close (physical) contact, e.g.

  • Meetings with friends
  • Sport
  • Services involving close contact (massage, physio etc.)

Objective

Backward contact tracing tries to identify the “source” of the virus that infected the index case and potentially to find other cases linked with this source in order to break chains of infection. This is done by recording specific significant events where COVID-19 exposure might have occurred.

Backward contact tracing vs. forward contact tracing

Unlike “classic” (forward) contact tracing, backward contact tracing tries to find out where somebody who tests positive today could have picked up the infection. Forward contact tracing tries to find out who somebody who tests positive today could have infected during their own infectious period. This is why backward contact tracing does not involve instructing people to quarantine. Backward contact tracing involves recording all relevant events during the past 14 days, while forward contact tracing involves recording all relevant contact persons during the 48 hours prior to onset of symptoms (or before a positive test in the case of asymptomatic individuals).

Model examples

The primary case in the diagram below is in the middle of the diagram (bright red). The case infects several people (index cases, dark red). Some of these people infect other people (secondary cases, orange). Not all primary or index cases lead to secondary cases. In order to break chains of infection, it is therefore important to identify specific primary cases or situations that lead to several infections. 

The next diagram shows a possible outcome of combining forward contact tracing and backward contact tracing. The index case (dark red) is known to the contact tracing team and is asked questions by them. Through backward contact tracing, the index case leads the tracers to a group of 5 attendees of an event. The primary case (bright red) infected the index case and a further 3 cases (yellow). A cluster was discovered here. Through backward contact tracing, the three newly discovered cases also lead to 5 further cases (yellow). The index case leads to two new cases through forward contact tracing (in orange). In this example, backward contact tracing found four more cases than forward contact tracing alone.