Immunogenicity and safety of LP.8.1 variant-containing mRNA COVID-19 vaccines

ABSTRACT Background The SARS-CoV-2 LP.8.1 subvariant was incorporated into the 2025-2026 U.S. COVID-19 vaccines (mRNA-1273.251 and mRNA-1283.251). We evaluated immunogenicity and safety of these vaccines against vaccine-matched and emerging variants in individuals aged ≥65 and those aged 12-64 years at high-risk of severe COVID-19. Methods Data were generated from: (1) two independent, ongoing, phase 3b/4, open-label, single-arm studies in which participants received a single dose of 50-µg mRNA-1273.251 (n=103; median age, 64.0 years) or 10-µg mRNA-1283.251 (n=172, median age, 59.0 years) and followed through Day 29 post-vaccination; neutralizing antibodies (nAb) were measured at baseline (Day 1) and Day 29 using a pseudovirus neutralization assay against the vaccine-matched LP.8.1 variant; (2) Day 29 immunogenicity against circulating variants (BA.3.2.2, XFG, and NB.1.8.1) was assessed in a randomly selected subset; and (3) immune-escape potential was estimated using predictive modeling. Unsolicited adverse events (AEs), including serious AEs, leading to study withdrawal, and those of special interest, were monitored. Results Both vaccines elicited robust nAbs at Day 29 against LP.8.1 (geometric mean fold-rise from baseline: 12-64 years, mRNA-1273.251, 26.3; mRNA-1283.251, 53.0; ≥65 years, mRNA-1273.251, 15.4; mRNA-1283.251, 36.7) and circulating variants. Model-based estimates with mRNA-1273.251 were consistent with clinical data and indicated the highest responses against LP.8.1 and lower responses against BA.3.2.2. No vaccine-related AEs were reported in either study. Conclusions mRNA-1273.251 and mRNA-1283.251 were well tolerated through Day 29 and elicited robust nAbs against vaccine-matched and circulating variants. In predictive models, BA.3.2.2 had the highest relative risk of immune escape following mRNA-1273.251 vaccination. SUMMARY LP.8.1-containing mRNA-1273.251 and mRNA-1283.251 vaccines given as a single dose were well tolerated and induced robust Day 29 neutralizing antibodies against LP.8.1 and circulating variants.