People wearing masks visit Asakusa in Tokyo on Sunday amid continuing worries over the new coronavirus Covid-19. Photo: AFP / The Yomiuri Shimbun

How should we regard the recent rise in Covid 19 cases in Japan? 

Reported new cases are still below 1,000 cases per day for the entire nation compared with over 70,000 in the United States. But the recent rise in number from a seven-day moving average low of 36 cases on May 27 is alarming.

Prime Minister Shinzo Abe sees no need to return to a national state of emergency as he decreed for the period between April 7 and May 25. Tokyo Governor Yuriko Koike says she is not sure if Tokyo is experiencing a “second wave.”

What wave two looks like

Consider the chart below. Reported new cases spiked above 900 per day recently, which is the highest ever recorded in Japan as shown with the blue line (left hand scale). Yes, higher than in April.

Daily cases here are calculated by the day-over-day increase in cumulative cases reported by the Health Ministry. The figure differs slightly from the aggregation of cases reported separately by each prefecture, which other news reports use.

The number of hospitalized patients, which was as low as 767 on June 20, soared to 5,635 as shown in the brown area (right hand scale). In this chart, the subjective criterion of “symptomatic patients” (green area) was switched to a more objective criterion of “hospitalized patients” (brown area) in May. 

Peak hospitalization rate during the first wave was around 6,500. That number is likely to be surpassed within the next 30 days if current trends continue.

Reproduction rate

After spending most of May in negative territory, the nationwide reproduction index spiked to 1.8 on July 6. 

That is not as high as the 2.2 recorded on April 1, a week before Prime Minister Abe declared a national emergency.  And the index has eased to 1.4 in recent days. 

But when the reproduction number is higher than 1.0, an infected person is likely infecting more than one person and the pandemic is spreading. When the reproduction number is less than 1.0, an infected person is not necessarily infecting others and the pandemic is disappearing.

If both the daily new case number and the hospitalization rate exceed those of the first wave, would that be enough to call the recent spike a second wave?

When daily new cases are setting a record, the hospitalization rate has not yet set but is likely to set new highs and the reproduction rate is again loitering above one, could we call that a second wave?

Positivity appears positive

One bright spot is that the test positivity rate remains below 4.0%.

When American states debate whether to reopen their schools, they often refer to a benchmark of 10% positivity rate. In Japan positivity has been trending down, from a peak of 9% in April to less than half that in recent days.

Testing is much more available than in April. Japan has averaged over 10,000 tests a day the past three weeks and has the capacity to triple this. Turnaround time for getting the results is shrinking and should be hours, not days, when new testers are in place. 

Growth rate accelerating

Compound daily growth rate of new cases was skidding along at 0.2% during most of May and June but has recently accelerated to 2.0%.

Compound daily growth rate over seven days is different from simple growth rate over seven days. If cases simply increase by 2.0% over 7 days, that means you start with 100 cases on Sunday and end up with 102 cases on Saturday. 

But with compound daily growth you have 102 cases on Monday, and 104 cases on Tuesday until you have 113 cases on Saturday.  So acceleration from 0.2% to 2.0% is a big deal in the world of compound growth such as in the financial markets.

Economists and financial analysts have been studying epidemiology since the market crash of March this year. Epidemiology studies are now part of financial and economic analysis. It is interesting to note that investment banks have begun to survey the percentages of people wearing masks in different economies. 

Yes, whether you wear a mask or not is now part of a nation’s credit rating that should impact the price of bonds and currencies.

A retired Tokyo-based analyst for a major US investment bank, Matt Aizawa now crunches numbers beside a lake north of the city.