Implication of backward contact tracing in the presence of overdispersed transmission in COVID-19 outbreaks

Abstract

Introduction: Contact tracing has the potential to control outbreaks without the need for stringent physical distancing policies, e.g. civil lockdowns. Unlike forward contact tracing, backward contact tracing identifies the source of newly detected cases. This approach is particularly valuable when there is high individual-level variation in the number of secondary transmissions (overdispersion). Methods: By using a simple branching process model, we explored the potential of combining backward contact tracing with more conventional forward contact tracing for control of COVID-19. We estimated the typical size of clusters that can be reached by backward tracing and simulated the incremental effectiveness of combining backward tracing with conventional forward tracing. Results: Across ranges of parameter values consistent with dynamics of SARS-CoV-2, backward tracing is expected to identify a primary case generating 3-10 times more infections than average, typically increasing the proportion of subsequent cases averted by a factor of 2-3. The estimated number of cases averted by backward tracing became greater with a higher degree of overdispersion. Conclusion: Backward contact tracing can be an effective tool for outbreak control, especially in the presence of overdispersion as was observed with SARS-CoV-2.

Superspreading in early transmissions of COVID-19 in Indonesia

“…2 ), suggesting that a small number of infected individuals are responsible for large amounts of the disease transmission. Our finding is consistent with other researcher has found in China 9 . Indeed, between 10–15% of all infections were responsible for 80% of onward transmission events.…”

Disease control across urban–rural gradients

“…While our strategic modelling approach accounts only for ‘average' parameter values, possible sources of variation in the efficacy of control may include forward versus backward test-and-trace efficacy (i.e. tracing known social contacts versus previous contact from which the infection has originated [ 34 ]), disproportionate disease spread by super-spreading individuals and super-spreading events, and dynamic changes in compliance by the public.…”

Perfect as the enemy of good: tracing transmissions with low-sensitivity tests to mitigate SARS-CoV-2 outbreaks

“…With backward contact tracing, when an index case is identified, the tracers attempt to identify the source of infection (either an infected individual or a transmission event), and then forward trace from that source to identify new infections and halt transmission by immediately isolating all contacts. 18 , 19 This strategy has several advantages, especially for a virus such as SARS-CoV-2, which transmits in clusters, and which many infected individuals do not transmit at all. 20 , 21 , 22 , 23 For such a pathogen, it is important to identify the source of infection when not known because there is a high probability that an infected invididual who has transmitted the virus at least once will transmit multiple times.…”

“…Various modelling and simulation studies have assessed these strategies individually in different settings, and we refer interested readers to these studies for more details. 6 , 7 , 8 , 9 , 10 , 11 , 14 , 16 , 17 , 18 , 24 , 27 , 28 , 29 , 30 , 33 , 36 , 37 , 38 , 39 , 40 , 41 , 42 We summarise selected strategies for control, noting some of their advantages and disadvantages ( table ). In particular, we consider the effect of overdispersed transmission and the presence of superspreading events in this comparison.…”