Age-Dependent Effects of COVID-19 Vaccine and of Healthcare Burden on COVID-19 Deaths, Tokyo, Japan

Abstract

COVID-19 vaccine effectiveness against death in Japan remains unknown. Furthermore, although evidence indicates that healthcare capacity influences case-fatality risk (CFR), it remains unknown whether this relationship is mediated by age. With a modeling study, we analyzed daily COVID-19 cases and deaths during January-August 2021 by using Tokyo surveillance data to jointly estimate COVID-19 vaccine effectiveness against death and age-specific CFR. We also examined daily healthcare operations to determine the association between healthcare burden and age-specific CFR. Among fully vaccinated patients, vaccine effectiveness against death was 88.6% among patients 60-69 years of age, 83.9% among patients 70-79 years of age, 83.5% among patients 80-89 years of age, and 77.7% among patients >90 years of age. A positive association of several indicators of healthcare burden with CFR among patients >70 years of age suggested an age-dependent effect of healthcare burden on CFR in Japan.

Keywords: 2019 novel coronavirus disease; COVID-19; Japan; SARS-CoV-2; case-fatality risk; coronavirus disease; healthcare burden; respiratory infections; severe acute respiratory syndrome coronavirus 2; vaccine effectiveness against death; vaccines; viruses; zoonoses.

Figure 1

Effects of COVID-19 vaccine on deaths, stratified by age group, Tokyo, Japan, January–August 2021. A) Daily number of SARS-CoV-2–positive case-patients (bar) and proportion of fully vaccinated SARS-CoV-2–positive case-patients (line, 7-day moving average). B) Daily number of deaths (bar) and proportion of deaths among fully vaccinated case-patients (line, 7-day moving average). Ranges indicate years of age. From top to bottom, age groups are 30–59, 60–69, 70–79, 80–89, >90 years.

Figure 2

Cumulative number of COVID-19 deaths in counterfactual scenarios (i.e., all positive case-patients vaccinated, all positive case-patients unvaccinated, and actual reported cases), Tokyo, Japan, January 1–August 31, 2021. A) 30–59; B) 60–69; C) 70–79; D) 80–89; E) >90 years of age. Shaded areas represent 95% credible intervals

Figure 3

Comparison of COVID-19 CFR predictions based on indicators of healthcare burden and asymptomatic rate with CFR when jointly estimated with VE, Tokyo, Japan, January 1–August 31, 2021. Shaded areas show 95% credible intervals. A) 30–59; B) 60–69; C) 70–79; D) 80–89; E) >90 years of age. CFR, case-fatality risk; VE, vaccine effectiveness.