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 a randomly chosen case, 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 is observed with SARS-CoV-2.

Keywords: COVID-19; SARS-CoV-2; backward tracing; contact tracing; overdispersion.

Figure 1.. Schematic illustration of forward and backward contact tracing.

Two cases (index cases #1 and #2) from a transmission tree originating from an (initially) undetected primary case are assumed to be detected by surveillance. Possible results of contact tracing are shown where ( A) only forward tracing is performed or ( B) both forward and backward tracing are performed. Some cases may remain undetected because contact tracing can miss cases.

Figure 2.. The estimated proportion of generation-3 (G3) cases averted by forward and backward contact tracing for different parameter values.

Left panels ( A, D, G): the effectiveness (the proportion of G3 cases averted) of forward tracing alone; middle panels ( B, E, H): the effectiveness of a combination of forward and backward tracing; right panels ( C, F, I): incremental effectiveness by combining backward tracing with forward tracing. Colours represent the relative reduction in transmission from G2 cases if traced and held in quarantine ( c). R: the reproduction number; k: overdispersion parameter; q: proportion of secondary infections caused by a detected case successfully traced; b: probability of successful identification of the primary case; d: probability of detection of generation-1 (G1) cases independent of contact tracing.

Figure 3.. The estimated absolute number of generation-3 (G3) cases averted by forward and backward contact tracing.

Left panels ( A, D, G): the number of cases averted by forward tracing alone; middle panels ( B, E, H): the number of cases averted by a combination of forward and backward tracing; right panels ( C, F, I): additional cases averted by combining backward tracing with forward tracing. Colours represent the assumed reproduction number R. k: overdispersion parameter; q: proportion of secondary infections caused by a detected case successful traced; c: relative reduction in transmission from quarantined cases; b: probability of successful identification of the primary case; d: probability of detection of generation-1 (G1) cases independent of contact tracing.