JOURNAL OF BACTERIOLOGY AND VIROLOGY ISSN:1598-2467(Print) 2093-0429(Online)

The relative and combined effects of non-pharmaceutical interventions (NPIs) and vaccination on SARS-CoV-2 transmission remain incompletely quantified, particularly in East Asian settings characterized by high policy compliance. We analyzed the epidemic trajectory in South Korea from May 2020 to July 2022 using a Bayesian hierarchical framework that incorporated variant-weighted transmissibility, NPI stringency indices, vaccine efficacy-adjusted coverage, and humidity as a control variable. Time-varying reproduction numbers were estimated and compared with EpiEstim-derived values for validation. Variant-specific weights were applied to account for shifts from the ancestral strain to Delta and Omicron lineages. Vaccination impact was modeled using product-specific efficacy and coverage proportions to reflect practical transmission reduction. Prior to vaccine rollout, NPIs accounted for a 55.5-77.8% reduction in transmission. Following vaccination initiation in early 2021, NPI effectiveness remained stable (68.6-77.8%), while vaccination effects increased progressively, reaching 54.5% by July 2022. The combined effect of NPIs and vaccination reached 91.7%, substantially exceeding their isolated contributions. Unlike observations in several European countries, NPI effectiveness in South Korea did not markedly decline after vaccine deployment. Humidity exerted a modest seasonal influence, and residual factors showed high uncertainty. Model robustness was confirmed through MCMC convergence diagnostics and PSIS-LOO validation. These findings demonstrate that sustained NPIs and vaccination functioned complementarily rather than competitively in reducing SARS-CoV-2 transmission. The results highlight the importance of integrated public health strategies and suggest that premature relaxation of NPIs following vaccine introduction may compromise epidemic control.