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Effective reproductive number must drop below 0.5 in Tokyo, 0.7 nationwide: exert panel

  • May 2, 2020
  • , Sankei , p. 2
  • JMH Translation

What did the state of emergency declaration and the prolonged stay-at-home measures accomplish? The government expert panel held a meeting on May 1 and focused not only on the number of newly reported new coronavirus cases, but also on the “effective reproductive number” — the number of people one person with the virus infects during an epidemic.


If the effective reproductive number is greater than 1, that means the virus is still spreading. If smaller than 1, the epidemic is subsiding. In the U.S. and Europe, the number is used as a rough guide for whether or not to loosen restrictions placed on people’s activities.


According to the expert panel’s proposal on May 1, the effective reproductive number was 2 nationwide in Japan on March 25, before the declaration of the state of emergency. It went down to 0.7 by April 10, after the emergency declaration. In Tokyo, the number was 2.6 on March 14, when the number of cases was starting to rise. This number was similar to that reported in Europe and the U.S. when the virus was spreading widely. After that, with a gubernatorial request to stay-at-home, the rate of increase of the new cases in Tokyo dropped to 0.5 by April 10.


Professor Hiroshi Nishiura of Hokkaido University, an expert on theoretical epidemiology, said, “We need to keep updating the data regularly to ensure that we maintain the level of 0.5 and then lower it.” Nishiura is a member of the Ministry of Health’s cluster team, which conducted a simulation study that led to the recommendation to reduce social contact by 80%.


The “basic reproductive number,” on the other hand, is the virus’ natural ability to infect. It is the number of people one individual with the virus infects under a normal, non-epidemic circumstance. The WHO estimates that the provisional basic reproductive number for the new coronavirus is somewhere between 1.4 and 2.5.

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