In Japan's coronavirus outbreak, numbers we know and don't know

Each bite of information illuminates a different aspect of the situation, but in pursuing a better view of the whole picture, some numbers might matter more than others.

The movement and trajectory of the contagion is best ascertained by measuring the increase in cases over a short period of time, possibly one or two weeks, said Koji Wada, professor of public health at the International University of Health and Welfare.

Wada is referring to the Rt, or the real-time effective reproductive ratio, where R is the actual transmission rate at a given time (t).

If R is around 1, it means an infected individual is transmitting the virus on average to one other person. If R dips below 1, the contagion will gradually subside. If it is over 1, the outbreak is growing.

R can vary based on location. It will likely be higher in confined spaces like an apartment building or cruise ship, and lower in rural villages with small populations.

In any case, the goal of every infected country at the moment is to bring R below 1 and keep it there.

Mathematical epidemiology is often lost on politicians, much less their constituents, due to vague or misleading verbiage. German Chancellor Angela Merkel, however, who holds a doctorate in physics, was praised for her acumen earlier this month when she explained what R actually means.

If the R in Germany reaches 1.2, Merkel explained, “out of five people, one infects two and the rest one.”

According to the government’s expert panel, the R in Tokyo between March 21 and 30 was approximately 1.7.

At the time, the capital had started to experience an alarming increase in cases. While the trend continued to escalate well into the middle of April, the contagion seems to have lost momentum over the past week.

The health ministry announced Japan’s R for March on April 1. But it did not release any of the data used to make the calculation or provide more recent estimates for Tokyo or anywhere else in the country.