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Tracking an infectious disease isn’t easy.

Whether it’s a smartphone, newspaper or television, facts and figures seem to leap from every surface one comes across.

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.

While the media naturally concentrate on changes in infections and deaths, public officials are focused on economic losses, the availability of hospital beds and how to reduce the movement of people in urban areas.

COVID-19 patients are described with an oftentimes confusing array of overlapping categories: positive, negative, tested negative then positive, hospitalized, experiencing minor symptoms, experiencing severe symptoms, in critical condition, quarantined at home, discharged and fully recovered.

Matters are complicated further by the need to account for age, gender, occupation, nationality and pre-existing health conditions.

While the sheer amount of information might be overwhelming for some, the numbers we don’t know — or can’t possibly know — are just as important, said Kenji Shibuya, director of the Institute of Public Health at King’s College London and a senior advisor to the director-general of the World Health Organization.

To understand the damage caused by the pandemic, Shibuya pointed to “excess mortality” or “mortality displacement,” which denotes a temporary increase in the death rate in a given population and is typically attributed to environmental phenomena, famine, war or infectious diseases.

But he said the biggest concern is the number of asymptomatic and pre-symptomatic transmissions, both of which are effectively impossible to measure without further testing and open data.

“Without public availability of data, we can’t say anything,” Shibuya said. “It’s important for the government and experts to share that kind of data set so we can understand what’s happening.”

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