Author: Gardner, B.; Kilpatrick, A. M.
Title: Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics Cord-id: rd8xazlz Document date: 2020_9_7
ID: rd8xazlz
Snippet: Simultaneously controlling COVID-19 epidemics and limiting economic and societal impacts presents a difficult challenge, especially with limited public health budgets. Testing, contact tracing, and isolating/quarantining is a key strategy that has been used to reduce transmission of SARS-CoV-2, the virus that causes COVID-19. However, manual contact tracing is a time-consuming process and as case numbers increase it takes longer to reach each cases' contacts, leading to additional virus spread.
Document: Simultaneously controlling COVID-19 epidemics and limiting economic and societal impacts presents a difficult challenge, especially with limited public health budgets. Testing, contact tracing, and isolating/quarantining is a key strategy that has been used to reduce transmission of SARS-CoV-2, the virus that causes COVID-19. However, manual contact tracing is a time-consuming process and as case numbers increase it takes longer to reach each cases' contacts, leading to additional virus spread. Delays between symptom onset and being tested (and receiving results), and a low fraction of symptomatic cases being tested can also reduce the impact of contact tracing on transmission. We examined the relationship between cases, delays, and participation and the pathogen reproductive number Rt. We also examined implications for infection dynamics using a stochastic compartment model of SARS-CoV-2. We found that Rt increases sigmoidally with the number of cases due to decreasing contact tracing efficacy. This relationship results in accelerating epidemics because Rt increases, rather than declines, as infections increase. Shifting contact tracers from locations with high and low case burdens relative to capacity to locations with intermediate case burdens maximizes their impact in reducing Rt (but minimizing total infections is more complicated). We also found that contact tracing quickly becomes ineffective in reducing Rt with increasing delays between symptom onset and tracing and with lower fraction of symptomatic infections being tested. Finally, we found that when cases are low, testing and tracing reductions in Rt can sometimes greatly delay epidemics due to the highly heterogeneous transmission dynamics of SARS-CoV-2, in which a small fraction of infections often give rise to most of transmission. These results demonstrate the importance of having an expandable or mobile team of contact tracers that can be used to control surges in cases. They also emphasize the value of easy access, high testing capacity and rapid turn-around of testing results, as well as outreach efforts to encourage symptomatic infections to be tested immediately after symptom onset. An efficient and adaptive public health capacity strategy can allow for increased economic activity and should be employed in the current and future pandemics.
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