**Abstract** What is the effect of social isolation policies on the new coronavirus dissemination? Social isolation policies rose to prominence as more capable of containing contagion and saving lives. Our purpose in this paper is to identify the causal effect of social isolation policies on the number of confirmed cases of Covid-19 and on contagion velocity. We align our main argument with the existing scientific consensus: social isolation policies negatively affect the number of contaminated cases. To test this hypothesis, we construct a dataset with daily information on 78 affected countries in the world. We compute several relevant measures from publicly available information on the number of infected cases and deaths in order to estimate causal effects for short-term and cumulative effects of social isolation policies. We use a time-series cross-sectional matching approach to match countries’ observable histories. Causal effects (ATTs and ATEs) can be extracted via a dif-in-dif estimator. Results show that social isolation policies reduce the aggregated number of contaminated people by 4,506 on average (or 17.11/100 thousand), but only when strict measures are adopted. This effect seems to manifest from the third week onwards. **Key Words**: Covid-19; government response; social isolation policies.