Merrymakers crowd in the Kabukicho nightlife district in Tokyo - a stark contrast to the misery and ruins of 1945. Photo: AFP / The Yomiuri Shimbun

Two weeks after the partial lockdown was lifted on June 19, case numbers are rising again in Japan. Clusters have been traced to nightlife activities in central Tokyo and the average age of cases is now below 35.

The seven-day moving average of new cases jumped to 120 on July 3, when 194 cases were reported. Two weeks earlier the moving average was less than 50.

Compared with the over 50,000 new cases per day in the United States, Japan’s number is a mere rounding error. But countries that have been successful in tamping down the pandemic have taken care not to miss such troubling trends.

Reproduction number up

Most troubling is the nationwide reproduction number, which says the pandemic is spreading again. That rate jumped to 1.6, a level last seen in early April when the lockdown was announced.

When the reproduction number is higher than 1.0, an infected person has 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.

Hospitalization above 1,200

As a result, the number of hospitalized patients nearly doubled to 1,223 from the low of 767 reported on June 20. Again the number is not as alarming as in early May when it peaked around 6,500 and it is minuscule compared with current US numbers. But when it comes to pandemics, it is the trend that matters.

Daily new cases are now up to the early May level. In the chart below, the subjective criterion “symptomatic patients” (green area) was switched to a more objective criterion, “hospitalized patients” (brown area), in May. 

Positivity down to 4%

Testing count remains low but positivity continues to decline. Japan has tested 381 persons per 100,000, which is about a tenth of what Korea has done. But the percentage of tests that are positive has declined from a high of 9% in mid-April to 4% most recently. While the reproduction rate has risen, the positivity rate declines as more testing capacity is added.

The new normal?

Like the slow burn of embers, the growth rate of cases has remained below 1.0% since May. But infections could flare up explosively within days, because asymptomatic carriers can transmit the virus.

Estimates of the number of asymptomatic carriers range widely. The percentage of those actually tested but without symptoms was around 8%, according to the Health Ministry. That roughly calculates to around 0.01% of the population. 

But antibody testing by various hospitals suggests that close to 0.10% of the population falls into the asymptomatic carriers category – 10 times the former estimate.

In the new normal we must consider the possibility that there are thousands, indeed tens of thousands of asymptomatic carriers, riding the subway with us, shopping in supermarkets and going to the office. We will just have to stock up on face masks for the foreseeable future.

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

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