Cluster surveillance using active epidemiological surveys, such as interviewing infected people, is also common abroad, but there is a big difference in the method between Japan and them. Japan has conducted two types of surveys: a "prospective contact tracing", which identifies close contacts with infected people starting with new cases, and "retrospective contact tracing", which identifies where new patients were infected and identifies close contacts together with them at their common source of infection. However, most countries have not carried out this "retroactive contact tracing" though some cases were confirmed in Taiwan.