The Mainichi Shimbun answers some common questions readers may have concerning Tokyo's coronavirus infection figures.
Question: Numerous cases of coronavirus infections have been confirmed in Tokyo, but why do figures vary greatly on different days?
Answer: The number of new infections in a day tends to fall during the first half of the week, and rise during the latter half. The Tokyo Metropolitan Government announces tallies based on reports submitted by medical institutions when they confirm infection cases, and it takes about three days from the time tests are conducted to when new cases are announced. Therefore, figures remain low in the first half of the week as cases from the weekend, during which hospitals are generally closed and few tests are conducted, are tallied during this period. On the contrary, figures announced later in the week include infections confirmed after those who stayed put at home during the weekend visited hospitals earlier in the week. Hence, a gap is seen among figures throughout the week.
Q: Why is there such a time lag between conducting the tests and announcing the results?
A: It takes one to two days to obtain results from the virus-detecting polymerase chain reaction tests. A time lag occurs as medical institutions then create reports on new infections and fax them to public health centers in the capital, and the metropolitan government compiles the data sent in by each public health center.
Q: What exactly are the news flashes on TV and other sources that appear every day around 3 p.m.?
A: The official announcement of coronavirus infections is issued at night when a metropolitan government official provides information to the press including the age brackets of those infected, as well as the main infection routes. The deadline for each public health center to submit reports on new infection cases to the metropolitan government is set at 9 a.m. each day, and the metro government goes about analyzing the data in preparation for the official announcement at night. There have been cases where news flashes had been issued after Tokyo Gov. Yuriko Koike responded to interviews around noon. Since late July, however, the metropolitan government itself has been announcing preliminary figures at 3 p.m.
Q: The reports that do not make it in time for the 9 a.m. deadline will be put off until the next day, so isn't this reporting method ineffective in grasping an accurate trajectory of coronavirus cases?
A: Exactly. We can't learn about the accurate transition of the current state of infections just by looking at the number of new "daily" cases announced by the metropolitan government each day, as explained thus far in this article. To understand the current situation more accurately, the national government's coronavirus advisory subcommittee uses data on the days patients developed symptoms.
On the metro government's website, figures for COVID-19 cases in Tokyo have also been classified to show the number of daily infections that were confirmed by the day test results were obtained, as well as the number of infected individuals by the day they showed their first symptoms. There are cases where set up in response to the novel coronavirus uses the latter type of data in its analyses in order to accurately grasp infection trends.
Detailed information, such as the data on dates that infected people developed symptoms, the ratio of people testing positive for the coronavirus and the number of tests conducted, are also available on the metropolitan government's website, so for those who wish to know more, it's possible to obtain further information there.
(Japanese original by Koichi Uchida, City News Department)