COVID-19 vaccination commenced globally in December 2020. Japan launched its vaccination rollout on February 17, 2021 and commenced booster vaccination campaign on December 1, 2021. It has been crucial to grasp the immune landscape in the country in order to aid in decision-making and evaluation of vaccination campaigns as well as understating the transmission dynamics of various variants of SARS-CoV-2. The present article shows a framework that enables us to predict the immune landscape, specifically, the proportion of immune population, using a mathematical modeling approach. This involved: prediction of vaccine coverage; estimation of vaccine effectiveness against the dominant SARS-CoV-2 variant in circulation; the quantification of increasing vaccine effectiveness (immune-build up) since receiving the first dose; the estimation of waning rate of vaccine effectiveness since receiving the second and third doses; and the consideration on the infection-induced immunity.