The quiet Shibuya District in Tokyo amid the Covid-19 outbreak. Photo: AFP

The US and Japan had about the same number of Covid-19 cases on March 6: 311 and 348, respectively.

Ten days later, cases rose to 4,509 in the US while Japan reported only 809. Five days later the numbers of cases were 23,661 vs. 996. What is going on?

There has been much speculation about cultural differences. One is a handshaking, hugging society while the other simply bows.

There is even speculation that Japan misreported the numbers to save the Olympic games. Now that the games have been postponed, the mayor of Tokyo has become openly alarmist, it is noted.

Epidemiologists are still wondering what crucial steps Japan took or didn’t take.

The answer may be the nationwide network of medical detectives at the National Institute of Infectious Diseases. Their daily briefing papers reveal a lot about infectious disease surveillance in Japan.

Doctors are legally required to report all cases of infectious diseases to NIID, which immediately establishes a case number and sets up a surveillance team of medical detectives to track all those who had close encounters with the patient.

When a man in his thirties living in Kanagawa was diagnosed as a carrier on January 16, the medical detectives identified, isolated and used the standard polymerase chain reaction test on 38 people.

To date NIID has focused PCR testing on such pinpoint targets instead of performing mass testing as done elsewhere. Korea had to resort to mass testing because on day two of its first diagnosis, there was credible evidence of 5,000 close encounters at a church.

By March 27, Japan had tested 27,005 individuals. Among them, 1,387 proved positive, among whom 148 individuals were asymptomatic; 1,212 were hospitalized, of whom 56 required ventilators and 331 have been discharged from hospitals.

Every one of these cases is closely monitored by NIID and the data are available online. Many of these cases have identified transmission points. The medical detectives know where and how the people got the virus. The transmission point, whether a theater or a gym, is shut down and sterilized.

A troubling trend emerged this past week. The medical detectives were finding cases that couldn’t be traced to a transmission point. What epidemiologists call “community spread” has begun. This is happening in New York City, where only 3% can be traced.

The earlier community spread in Hokkaido appears to be settling. But untraceable cases are emerging in Tokyo and Osaka. This is why the governor of Tokyo issued a stay-home warning last week. The postponement of the Olympic games was likely coincidental.

Pinpoint testing will continue. But if cases of community spread increase, Japan will have to engage in mass testing as well.

So what did Japan do that distinguishes its case numbers from the US?  Probably better surveillance, meticulous medical detective work. You can see it daily at the Ministry of Health, Labor and Welfare home page.