Projecting the COVID-19 immune landscape in Japan in the presence of waning immunity and booster vaccination

Coronavirus disease 2019 (COVID-19) booster vaccination has been implemented globally in the midst of surges in infection due to the Delta and Omicron variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The objective of the present study was to present a framework to estimate the proportion of the pop-ulation that is immune to symptomatic SARS-CoV-2 infection with the Omicron variant (immune proportion) in Japan, considering the waning of immunity resulting from vaccination and naturally acquired infection. We quantified the decay rate of immunity against symptomatic infection with Omicron conferred by the second and third doses of COVID-19 vaccine. We estimated the current and future vaccination coverage for the second and third vaccine doses from February 17, 2021 to August 1, 2022 and used data on the confirmed COVID-19 incidence from February 17, 2021 to April 10, 2022. From this information, we estimated the age-specific im-mune proportion over the period from February 17, 2021 to August 1, 2022. Vaccine-induced immunity, conferred by the second vaccine dose in particular, was estimated to rapidly wane. There were substantial variations in the estimated immune proportion by age group because each age cohort experienced different vaccination rollout timing and speed as well as a different infection risk. Such variations collectively contributed to heterogeneous immune landscape trajectories over time and age. The resulting prediction of the proportion of the population that is immune to symptomatic SARS-CoV-2 infection could aid decision-making on when and for whom another round of booster vaccination should be considered. This manuscript was submitted as part of a theme issue on Modelling COVID-19 and Preparedness for Future Pandemics.

Automated summary

The study aimed to estimate the proportion of the population in Japan immune to symptomatic SARS-CoV-2 infection with the Omicron variant, considering waning immunity from vaccination and natural infection. By quantifying the decay rate of Omicron immunity conferred by COVID-19 vaccine doses and analyzing vaccination coverage and COVID-19 incidence data, the study estimated age-specific immune proportions over a specific period. The findings highlighted the rapid waning of vaccine-induced immunity, particularly with the second dose, and substantial variations in immune proportions by age group.