TOKYO -- If Japan's coronavirus state of emergency were to be lifted when the number of daily new cases had dropped to below 500, it could be back to the surging infection levels seen pre-emergency declaration in less than 50 days, an estimate by a theoretical epidemiology professor has shown.
Kyoto University professor Hiroshi Nishiura who specializes in epidemiological analysis released his calculation on Jan. 13. The Japanese government has mentioned that one of the indicators for Tokyo to lift the coronavirus state of emergency was if the number of new infections per day hit fewer than 500. Nishiura warns that if the criteria to lift the emergency declaration are relatively lax, such as the under-500 mark, Japan "could see a total of three infection figure peaks, including the current one, by late July." He pointed out that a long-term outlook is necessary to decide on the criteria.
Professor Nishiura used "effective reproduction number," or Rt, which indicates the average number of people that can be infected by one virus carrier and forecast the timing of a coronavirus resurgence based on the state of infections in Tokyo. He assumed that the Rt figure in the capital was 1.1 based on data from around mid-December 2020. He then calculated the potential shifts in the number of new infections for two separate scenarios: one in which Rt falls by 20% due to the effectiveness of relatively lax measures and the other in which the figure drops 35% with stricter measures.
His theory first assumed that in both scenarios Rt remained at 1.1 as the number of infections would increase after the state of emergency was lifted due to weakened measures in the post-emergency period. Under this hypothesis, the number of daily new cases dips below 500 on Feb. 24 in the lax measure scenario. If the state of emergency is to be lifted then, the figure increases to some 1,000 per day -- the pre-emergency declaration level -- by mid-April. For the stricter measure scenario, meanwhile, the indicator to lift the state of emergency was set at "100 new cases per day," and Nishiura calculated with the assumption that the declaration continued even if the daily cases dropped below 500. In this case, the number of daily infections falls to fewer than 100 on Feb. 25 and would not get back to the pre-state of emergency level until mid-July.
Nishiura also conducted simulations based on the assumption that people's social activities quickly resumed following the lifting of the state of emergency and the Rt hiked to 1.43. In this hypothesis, it only takes 25 days from Feb. 25 -- where the daily cases drop below 100 in the stricter measure scenario -- for the figure to bounce back to the level in which yet another state of emergency declaration is necessary.
Based on the state of infection indicators compiled by a coronavirus subcommittee, the Japanese government envisages a situation where it lifts the state of emergency when the country sees its current outbreak status, which in some prefectures is at "stage 4," meaning an explosive spread of infections -- the worst of the four-stage indicators showing the state of coronavirus infections in Japan -- goes to stage 3, which indicates a sharp increase in the number of infections. Regarding this, professor Nishiura commented, "The government has said it would continue to take measures to aim for stage 2 (a gradual increase in infections) even if the state of emergency was lifted at stage 3. I wonder why they don't just keep the declaration in place until stage 2."
Nishiura also emphasized that regarding lifting the state of emergency, social and economic activities "need to resume gradually" while paying attention to the state of infections.
(Japanese original by Sooryeon Kim, Lifestyle and Medical News Department)