{Excerpt}
In the spring of 2025, multiple SARS-CoV-2 Omicron JN.1 subvariants were circulating, with LP.8.1 among the major variants. Pharmaceutical companies, such as Pfizer–BioNTech, Moderna, and Novavax–Takeda, adopted monovalent LP.8.1 for their 2025–26 season vaccines, following recommendations issued by WHO in May, 2025. As of November, 2025, SARS-CoV-2 variants including LP.8.1, XEC, NB.1.8.1, and XFG—all designated as variants under monitoring—were circulating. In terms of the spike gene, these variants, as well as LP.8.1, are derived from JN.1. Moreover, BA.3.2, a BA.3 descendant bearing multiple mutations in its spike gene, has potentially been spreading and exhibiting robust immune evasion. In Japan, the roll-out of the LP.8.1-based vaccination has progressed since the end of September, 2025. We previously reported the humoral immunity induced by the XBB.1.5-based monovalent vaccine in 2023,6 and the JN.1-based monovalent vaccine in 2024 in the Japanese population. We investigated the efficiency of humoral immunity induced by two LP.8.1-based vaccines, the mRNA vaccine from Pfizer–BioNTech and the recombinant protein-based vaccine from Novavax–Takeda, in Japan. Of note, the Novavax SARS-CoV-2 vaccine strain was updated to LP.8.1 only in Japan in 2025, whereas the formulation remains unchanged from JN.1 globally outside of Japan.
