VaccineNov 12, 2025

Immune response and safety of a single-part XBB.1.5 COVID-19 vaccine in Japanese people

mRNA Technology Weekly Brief ↗PubMed ↗DOI ↗

Updated Jun 28, 2026

Abstract

Essence

In Japanese participants aged 12 years or older, the XBB.1.5 monovalent mRNA-1283 vaccine produced neutralizing antibody responses that were at least as strong as mRNA-1273 with no new safety concerns.

Evidence

This phase 3 randomized observer-blinded study in 689 participants found Day 29 neutralizing antibodies against Omicron XBB.1.5 were noninferior for 10 microg mRNA-1283.815 versus 50 microg mRNA-1273.815, with a geometric mean ratio of 1.195 (95% CI 1.028-1.389).

Caveat

The abstract reports short-term immunogenicity and safety through Day 29, not clinical protection or longer-term outcomes.

Simplified

mRNA-1283 is a next-generation COVID-19 vaccine encoding the immunodominant N-terminal and receptor-binding domains of the SARS-CoV-2 spike protein. This phase 3 randomized, observer-blinded study compared the immunogenicity and safety between monovalent Omicron XBB.1.5-containing formulations of mRNA-1283 (mRNA-1283.815; 10 μg) and mRNA-1273 (mRNA-1273.815; 50 μg) in Japanese participants (≥12 years). The primary objectives were to assess safety and demonstrate the noninferiority of neutralizing antibody response of mRNA-1283.815 versus mRNA-1273.815 against Omicron XBB.1.5 based on the geometric mean ratio (GMR) at Day 29 post-injection (lower bound of 95 % CI, >0.667). Overall, 689 randomly assigned participants received a study vaccine. mRNA-1283.815 was well-tolerated, with no safety concerns identified. mRNA-1283.815 elicited noninferior neutralizing antibody responses to mRNA-1273.815 against Omicron XBB.1.5 at Day 29, meeting the primary immunogenicity objective, with the GMR indicating higher responses of mRNA-1283.815 vs mRNA-1273.815 (GMR [95 % CI], 1.195 [1.028-1.389]). These results support the effectiveness of a monovalent formulation of mRNA-1283.