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.

SARS-CoV-2已在世界各地确立了自己的地位，许多国家已将社会隔离作为一种措施，以防止卫生保健系统负担过重。在这里，我们评估在各种社会距离的环境中，通过隔离和接触者追踪，是否可能以及在何种情况下可以遏制SARS-CoV-2。为此，我们使用分支过程模型，其中每个人都根据受潜伏期分布，潜伏期分布和传染性影响的概率分布来产生新型感染。该模型区分家庭和非家庭接触。社会距离可能会影响这两种类型的联系人的数量，例如，在减少工作和学校联系的同时，家庭联系人可能保持不变。该模型允许显式计算基本和有效繁殖数量，以及指数增长率和倍增时间。我们的发现表明，如果模型中无症状感染的比例大于30％，则接触者的追踪和隔离无法达到基本繁殖数的遏制（ℛ0）的2.5。要通过社会疏远来实现遏制，就需要将非家庭联系人数减少约90％。如果无法遏制，则至少有可能降低流行病的增长率和增加倍增时间。我们显示了各种参数组合，如何通过接触跟踪来降低增长率和增加两倍的时间。根据实现的减少接触的程度，只有将家庭接触者或家庭和非家庭接触者进行跟踪和隔离才能将有效复制数量降低到1以下。在社会疏远的情况下，接触者跟踪可以协同作用将秤推向密闭状态。