Japan’s unique way of contact tracing also gave us more clues into how the virus spread. While other countries focused on prospective contact tracing, in which contact tracers identify and notify infected people’s contacts after they are infected, we used retrospective contact tracing. This is an approach where tracers identify an infected person and look back to figure out when and where that person was infected and who else might have been infected simultaneously with them.

This approach turned out to be critical as we learned that the coronavirus was being spread predominantly by small numbers of infected individuals who then go on to seed super-spreading events. My research colleague Hiroshi Nishiura calculated that a majority of cases were most likely coming from infected people in closed, indoor environments. More data from public health centers in Japan confirmed that most Covid-19 clusters occurred in close-contact indoor settings, such as dinners, night clubs, karaoke bars, live music venues and gyms.

This has become common knowledge now, but we knew all of this before the end of February 2020 and before the World Health Organization considered Covid-19 a pandemic. This became the basis of Japan’s strategy going forward and is ultimately what allowed Japan to have one of the lowest death rates among its peer countries.