At a press conference on May 17, Minister in charge of Economic Revitalization Yasutoshi Nishimura said that he plans to include the “K value” as an indicator of COVID-19 trends in the government’s assessment of the domestic coronavirus situation. The K value indicates the rate of increase of the cumulative number of COVID-19 cases in the past week. A low K value means that the virus situation is getting under control. The K value is in the spotlight today, as it is not affected by day-to-day fluctuations.
A team led by Professor Takashi Nakano at the Osaka University Research Center for Nuclear Physics have advocated using the K value and presented the idea at a meeting of the government’s expert panel on May 14. To calculate the K value, the number of new infections over the past week is divided by the cumulative number of infections. A K value of 0.05, for example, means that the number of cases increased by 5% in that one week. According to Nishimura, using the cumulative number of cases since March 25, the K value for the whole nation on April 14, a week after the state of emergency was declared, was 0.55, which means that the number of the new COVID-19 cases in the previous week made up more than half of the cumulative number of cases. The K value as of May 16 was 0.035.
On May 6, Germany announced measures to ease restrictions on economic activities. The country’s K value on that day was 0.044. France lifted its restrictions on nonessential outings on May 11. The country’s K value that day was 0.059. Based on these figures, Professor Nakano considers 0.05 as a threshold. When 0.05 is applied to Tokyo Metropolis, the number of the new cases in a week would be 240 (1.7 person per 100,000). Japan’s K value as of May 16 was 0.042. “The K value would be of some help in assessing COVID-19 trends,” said Nishimura. “I will keep following the K value as one of our numerical indicators.”