The government on May 25 lifted the state of emergency declared over COVID-19 as the number of new infections with the novel coronavirus decreased to mere dozens per day. While the risk of the second wave is imminent, it seems that Japan, to a large extent, has successfully minimized COVID-19-related deaths without introducing a strict lockdown or a broad testing policy during what is considered to be the first wave of the pandemic. How was this possible?

The success in this first phase was largely underpinned by a proper understanding of the transmission dynamics through identifying clusters — groups of infected people from a single source — by finding case links through investigations by local public health centers.

Together with the medical staffs in local hospitals nationwide, the more than 25,000 public health workers at public health centers in all 47 prefectures who tracked down the infection routes are considered to be unsung heroes who successfully contained the threat posed by the virus.

While conducting contact investigations of the imported and related cases, we experienced difficulty in finding new cases from the contacts. This was mysterious given the high infection rate we had observed in the outbreak aboard the cruise ship Diamond Princess.

Further preliminary analysis of domestic cases revealed that in most cases a majority of patients did not infect others, but that a limited number of cases caused more than five secondary cases, forming clusters. Therefore we allocated resources to investigate these clusters to efficiently track down the unrecognized cases.

Especially in the early phase of the virus when the prevalence was extremely low, encouraging people with mild or no symptoms to take PCR tests would have revealed nothing but resulted in isolating false-positive cases. Through its strategic “cluster-focused approach,” the government was able to identify environmental risk factors and risk behavior that cause clusters.

The easy-to-understand slogan that cautioned the public against “closed, crowded spaces with close-contact (the three Cs)” was also an effective communication strategy. These efforts may have prevented clusters from forming and delayed the exponential growth in cases without damaging the economy by legally restricting the movement of people.

The cluster-focused approach also enabled the government to detect signs of exponential growth of cases at a very early stage, thereby allowing it to provide the public with an effective early warning.

For example, when an increase in the number of patients whom we were unable to follow source links was observed in Osaka and Tokyo from mid-March, these regions requested residents to stay at home on weekends in the second half of the month — a measure that proved to be effective. According to an analysis in the National Expert Committee’s Report on May 29, the peak of infection was identified to have taken place from late March to early April, which was not recognizable from the daily surveillance report at that time.

Even though the measures under the state of emergency were nonbinding, Japan managed to reduce the number of new infections, leading to the May 25 lifting of the state of emergency across the country. As we brace for the risk of a next spike, our goal remains the same: minimize the number of COVID-19 deaths with a cluster-focused approach while keeping the socio-economic impact to a minimum.

The challenge here is whether Japan can remain equally resilient during the expected second wave — remember, the government does not have the legal authority to implement strong measures such as lockdowns and curfews — and whether people can get used to a “new normal” by only exercising peer pressure, which is considered to be strong in Japanese culture.

We medics in Japan are prepared to do our utmost but I also cannot help but pray that the Japanese public will demonstrate its resilience equally in the second phase, which could happen any moment.

Tomoya Saito is director of the Department of Health Crisis Management at the National Institute of Public Health.