Abstract
SARS-CoV-2 has established itself in all parts of the world, and many countries have implemented social distancing as a measure to prevent overburdening of health care systems. Here we evaluate whether and under which conditions containment of SARS-CoV-2 is possible by isolation and contact tracing in settings with various levels of social distancing. To this end we use a branching process model in which every person generates novel infections according to a probability distribution that is affected by the incubation period distribution, distribution of the latent period, and infectivity. The model distinguishes between household and non-household contacts. Social distancing may affect the numbers of the two types of contacts differently, for example while work and school contacts are reduced, household contacts may remain unchanged. The model allows for an explicit calculation of the basic and effective reproduction numbers, and of exponential growth rates and doubling times. Our findings indicate that if the proportion of asymptomatic infections in the model is larger than 30%, contact tracing and isolation cannot achieve containment for a basic reproduction number (ℛ0) of 2.5. Achieving containment by social distancing requires a reduction of numbers of non-household contacts by around 90%. If containment is not possible, at least a reduction of epidemic growth rate and an increase in doubling time may be possible. We show for various parameter combinations how growth rates can be reduced and doubling times increased by contact tracing. Depending on the realized level of contact reduction, tracing and isolation of only household contacts, or of household and non-household contacts are necessary to reduce the effective reproduction number to below 1. In a situation with social distancing, contact tracing can act synergistically to tip the scale toward containment. These measures can therefore be a tool for controlling COVID-19 epidemics as part of an exit strategy from lock-down measures or for preventing secondary waves of COVID-19.
Highlights
The novel coronavirus (SARS-CoV-2) has established itself in all parts of the world
In the baseline scenario without interventions the exponential growth rate is 0.16 per day and the doubling time is 4.4 days, which agrees with published estimates [32, 33]
For lower values of R0, for example if reproduction numbers are reduced by social distancing, control is possible at longer delays and lower tracing coverages
Summary
The novel coronavirus (SARS-CoV-2) has established itself in all parts of the world. There are still no registered vaccines and treatment options to COVID-19 disease remain mainly supportive. Control of virus transmission and associated disease depends on preventive measures such as social distancing combined with isolation of infected persons and those that have high likelihood of being. It has become clear that additional measures are needed to control epidemic transmission, for example by using active tracing of contacts in combination with isolation of infected contacts. Such measures are important in the context of exit strategies, i.e. once social distancing measures are reduced or lifted, as has been suggested recently [3]. It is unclear how effective such combinations of interventions can be in populations with social distancing in place [4]
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