A population can be immune to epidemics even if not all of its individual members are immune to the disease, so long as sufficiently many are immune—this is the traditional notion of herd immunity. In the smartphone era a population can be immune to epidemics even if not a single one of its members is immune to the disease—a notion we call ‘digital herd immunity’, which is similarly an emergent characteristic of the population. This immunity arises because contact-tracing protocols based on smartphone capabilities can lead to highly efficient quarantining of infected population members and thus the extinguishing of nascent epidemics. When the disease characteristics are favorable and smartphone usage is high enough, the population is in this immune phase. As usage decreases there is a novel ‘contact-tracing phase transition’ to an epidemic phase. We present and study a simple branching-process model for COVID-19 and show that digital immunity is possible regardless of the proportion of non-symptomatic transmission.

一个群体即使不是所有的个体成员都对这种疾病免疫，只要足够多的人免疫，它也可以对流行病免疫——这是群体免疫的传统概念。在智能手机时代，即使没有一个成员对这种疾病免疫，人们也可以对流行病免疫——我们称之为“数字群体免疫”的概念，这同样是人口的一个新兴特征。之所以产生这种免疫力，是因为基于智能手机功能的接触者追踪协议可以高效隔离受感染的人群，从而消灭新生的流行病。当疾病特征有利且智能手机使用率足够高时，人群处于此免疫阶段。随着使用量的减少，出现了一种新的“接触者追踪阶段过渡”到流行阶段。我们提出并研究了 COVID-19 的简单分支过程模型，并表明无论无症状传播的比例如何，数字免疫都是可能的。