ABSTRACT
Background: The Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified in Japan in November 2021. This variant contains up to 36 mutations in the spike protein, the target of neutralizing antibodies, and can escape vaccine-induced immunity. The third booster vaccination campaign began with healthcare workers and high-risk groups. The safety and immunogenicity of third booster vaccination against Omicrons remain unknown. Methods: In total, 272 healthcare workers were evaluated for their long-term safety and immunogenicity. Here, we established vaccine panels to evaluate the safety and immunogenicity against variants of concern (VOCs), including the Omicron variant, using a live virus microneutralization assay. Findings: Two-dose vaccination induced robust anti-spike antibodies and neutralization titers (NTs) against the ancestral strain WK-521, whereas NTs in VOCs were significantly decreased. Within 93-247 days of the second vaccine dose, NTs against Omicron were completely abolished in up to 80% of individuals among the vaccine panels. The third booster vaccination induced a robust increase in anti-spike antibodies and NTs against the WK-521, Delta, and Omicron variants. The breadth of humoral immunity and cross-reactivity with Omicron increased. The cytokine signature and adverse event rate remained unchanged after three dose vaccination. Conclusions: The third vaccination dose is safe and effective against Omicron infection.