Effectiveness of isolation and contact tracing for containment and slowing down a COVID-19 epidemic: a modelling study

Mirjam E Kretzschmar; Ganna Rozhnova; Michiel E van Boven

Findings: The proportion of asymptomatic or unascertained cases has a strong impact on the 29 controllability of the disease. If the proportion of asymptomatic infections is larger than 30 30%, contact tracing and isolation cannot achieve containment for an R0 of 2.5. Achieving 31 containment by social distancing requires a reduction of numbers of non-household 32 contacts by around 90%. Depending on the realized level of contact reduction, tracing and 33 isolation of only household contacts, or of household and non-household contacts are 34 necessary to reduce the effective reproduction number to below 1. A combination of social 35 Research in context 46 Evidence before this study 47 As of 8 April 2020, the novel coronavirus (SARS-CoV-2) has spread to more than 170 48 countries and has caused near 90,000 deaths of COVID-19 worldwide. In the absence of 49 effective medicines and vaccines, the preventive measures are limited to social distancing, 50 isolation of confirmed and suspected cases, and identification and quarantining of their 51 contacts. Evidence suggests that a substantial portion of transmission may occur before the 52 onset of symptoms and before cases can be isolated, and that many cases remain 53 unascertained. This has potentially important implications for the prospect of containment 54 by combinations of these measures. 55 Added value of this study 56 Using a stochastic transmission model armed with current best estimates of epidemiological 57 parameters, we evaluated under which conditions containment could be achieved with 58 combinations of social distancing, isolation and contact tracing. We investigated the level of 59 social distancing needed for containment, and how an additional implementation of 60 isolation and contact tracing may likely help to in reducing the effective reproduction 61 number to below 1, the critical threshold. We analyzed what proportion of household and 62 non-household contacts need to be isolated effectively to achieve containment depending 63 on the level of social distancing in the population. We estimated the impact of combinations 64 of these measures on epidemic growth rate and doubling time for the number of infections. 5 household contacts is sufficient, depends on levels of social distancing and timeliness of 68 tracing and isolation. 69 Implications of all the available evidence 70 Our analyses based on best understanding of the epidemiology of COVID-19, highlight that 71 if social distancing is not complete, isolation and contact tracing at least of household 72 contacts can help to delay and lower the epidemic peak. High levels of timely contact 73 tracing of household and non-household contacts may be sufficient to control the epidemic. 74 Introduction 75 As of early April 2020, the number of infections of the novel coronavirus (SARS-CoV-2) is still 76 increasing at an exponential rate in many countries, while the virus is expanding its range to 77 all parts of the world. There are no registered effective medicines, treatment options are 78 mainly supportive, and there are no vaccines available, limiting preventive measures mainly 79 to social distancing combined with isolation of infected persons and those that have high 80 likelihood of being infected, for instance because they have been traced as contacts of 81 infected persons [1, 2] . However, recently it was questioned by high-ranked officials of the 82 World Health Organization (WHO) whether social distancing or lockd