An interesting study of the transmission is here. Even if the absolute numbers may be disputed the general message about the measures and their impact is interesting. Especially the role of time of response/application of the measures: https://science.sciencemag.org/conte...cience.abb6105

Of course with the first country it is difficult to set the day 1 since it takes time to find out that something new is happening but on the other hand in the other countries the first detected case also wasn't the real number 1 I guess. Anyway it's important to note that China introduced brutal measures at day 23 while US is at day 71 now and till day 60 or so they did exactly nothing. Even now the measures are nowhere close to the level of Chinese ones (quite impossible in the western world to be honest). UK is now in day 64. Here in CZ the social distancing measures were applied starting from day 13 (getting stricted during the following days).