Evaluating the COVID-19 vaccination program in Japan, 2021 using the counterfactual reproduction number

Abstract

Japan implemented its nationwide vaccination program against COVID-19 in 2021, immunizing more than one million people (approximately 1%) a day. However, the direct and indirect impacts of the program at the population level have yet to be fully evaluated. To assess the vaccine effectiveness during the Delta variant (B.1.617.2) epidemic in 2021, we used a renewal process model. A transmission model was fitted to the confirmed cases from 17 February to 30 November 2021. In the absence of vaccination, the cumulative numbers of infections and deaths during the study period were estimated to be 63.3 million (95% confidence interval [CI] 63.2–63.6) and 364,000 (95% CI 363–366), respectively; the actual numbers of infections and deaths were 4.7 million and 10,000, respectively. Were the vaccination implemented 14 days earlier, there could have been 54% and 48% fewer cases and deaths, respectively, than the actual numbers. We demonstrated the very high effectiveness of COVID-19 vaccination in Japan during 2021, which reduced mortality by more than 97% compared with the counterfactual scenario. The timing of expanding vaccination and vaccine recipients could be key to mitigating the disease burden of COVID-19. Rapid and proper decision making based on firm epidemiological input is vital.

Unreliability in Simulations of COVID-19 Cases and Deaths Based on Transmission Models

“…The two papers on counterfactual simulations of COVID-19 cases and deaths are based on an extended version of the SIR model. In the paper [1], the effective reproduction number, which is interpreted as the average number of secondary cases in age group a generated by a single primary case in age group b at calendar time t , is given by where l a,t denotes the immune fraction in age group a at calendar time t , which is based on the estimated efficacy of vaccines, while represents the cumulative number of previous infections. Note that left bracketed term in eqn.…”

“…To confirm whether the above data can be observed from the model by Kayano et al, we carried out the following test. Kayano et al [1] claim that COVID deaths would have decreased by 48% if the vaccination program had started 14 days earlier and would have increased by 50% if it had started 14 days later. This prediction includes deaths from February 17 to November 30.…”

“…Prediction curve based on the results by Kayano et al [1] (A) and the differences between the values of the prediction curve and the real-world data in Groups I (B), II (C), and III (D).…”

“…In October 2023, two papers on counterfactual simulations of COVID-19 cases and deaths using transmission models were published in two different academic journals. The first paper is titled "Evaluating the COVID-19 vaccination program in Japan, 2021 using the counterfactual reproduction number" [1] and the second paper is titled "Assessing the COVID-19 vaccination program during the Omicron variant (B.1.1.529) epidemic in early 2022, Tokyo" [2].…”

“…In this paper, we reexamine the credibility of the parameters Kayano et al used in the simulations [1,2] by checking the cited data and comparing the simulation results with the actual statistics, including prefectural data where vaccination programs were administered with different time schedules. We also discuss the implication of the result for public health policy making.…”