Students keeping distance and wearing face masks take part in an entrance ceremony at the Yokohama Municipal Nakayama Elementary School in Kanagawa Prefecture on Monday, amid further spread of the outbreak of Covid-19. The Tokyo Metropolitan Government announced that the daily increase surpassed 100 people for the first time on the previous day, which has raised pressure on Japan to declare a state of emergency. Photo: AFP / The Yomiuri Shimbun

As prime minister Shinzo Abe reportedly prepares to declare a national emergency, some of my fellow number crunchers say the number of known Covid-19 cases in Japan is approximately three weeks behind the US. But it’s not clear exactly what that means.

In January, all cases in Japan were traced, tested and isolated including those with close encounters. In February, a small number of cases could not be traced to known sources. In March, about half could not be traced. Now more than half can not be traced. But the medical detectives have not given up, they still continue to trace, test and isolate where they can.

By comparison, the United States has virtually given up on tracing, testing is not available even when warranted and community spread is everywhere. So extrapolating the trend in Japan based on the past three weeks in the US would be hazardous.

 
 
 Sources: Johns Hopkins, CDC and NYT, Date: April 4

 
 
 Source: Ministry of Health, Labour & Welfare, Date April 4

 Remember that discussion about log charts? If the log says 3, it means 10 to the third power or 10 x 10 x 10 or 1,000.  The log “speed” in Japan is around 3.5 while the log “speed” in the US is around 5.5.  So the two countries are traveling at hugely different speeds.

I attribute that difference in speed to differences in (a) persistence in tracing & testing, (b) nation-wide coordination and (c) different social norms. The US CDC only this week said to wear masks.

So I am still optimistic that Japan will not reach log “speed” of 5.0

In the meantime notice how the US log chart may be suggesting that the rate at which the transmissions have been accelerating is now slowing – maybe.

That means maybe the rate-of-change-of-rate-of-change (second differential) is not accelerating. Still too early to draw any conclusion.

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