News Navigator: Why is coronavirus death rate lower during 'second wave' in Japan?

The Mainichi Shimbun answers some common questions readers may have about the "second wave" of coronavirus infections that have been spreading in Japan since June, and why the fatality rate is lower compared to the "first wave" of infections during March and April.

Question: What have been the mortality rates in the first and second waves?

Answer: According to an estimation by Motoi Suzuki, head of the Infectious Disease Surveillance Center at the National Institute of Infectious Diseases, as of Aug. 30 the death rate of the first wave (from Jan. 16 to May 31) stood at 5.8% and the second wave (from June 1 to Aug. 19) stood at 0.9%. The fatality rate in people aged 70 and older was 24.5% in the first wave and 8.7% in the second wave. The mortality rate among all age groups was lower in the second wave.

Q: Why did the death rate go down?

A: The main reason is apparently due to a wider range of people becoming eligible for testing in the second wave, enabling the detection of more infections, including among asymptomatic patients and those with mild symptoms. The fatality rate is the number of accumulated deaths divided by accumulated infections, and greatly differs by what is included in the denominator. For example, if the denominator only consists of patients with severe symptoms, the result will be a larger figure, whereas if the denominator consists of all infections including asymptomatic cases, the result will be a smaller figure.

Q: What other reasons are there?

A: Treatment methods including those using medication have been established to some extent, which some have pointed out as a major reason for the reduced fatality rate. It is also believed that the spread of the virus among many young people, who are less inclined to develop severe symptoms, and infections in people who are relatively healthy among elderly patients are contributing factors.

Q: Has the virus also weakened?

A: There hasn't been any evidence found showing the virus has weakened yet. We are now seeing a more accurate, larger picture of coronavirus infections by broadening the view on patients in the second wave. Though a lower mortality rate is a good thing, anyone can still become infected and possibly develop severe symptoms. It's crucial to take preventative measures such as wearing masks and avoiding the "three Cs" of confined spaces, crowded places and close contact.

(Japanese original by Sooryeon Kim, Lifestyle and Medical News Department)