Interim analysis of effectiveness of JN.1-adapted conventional and self-amplifying mRNA COVID-19 vaccines against symptomatic SARS-CoV-2 infection in adults aged ≥60 years, Japan, October 2024–April 2025

In Japan, JN.1-adapted conventional mRNA and self-amplifying mRNA (sa-mRNA) COVID-19 vaccines were introduced during the 2024–2025 season. We estimated the effectiveness of these vaccines against symptomatic SARS-CoV-2 infection among adults aged ≥60 years using a test-negative design (October 1, 2024–April 30, 2025). Eligible episodes were ≥1 COVID-19-like symptom and SARS-CoV-2 testing. Vaccine effectiveness (VE) was estimated using mixed-effects logistic regression. We included 954 episodes; 273 (28.6%) were SARS-CoV-2-positive. VE of combined JN.1-adapted conventional and self-amplifying mRNA vaccines was 46.7% (95% CI: 5.5–69.9) at ≥7 days, 51.6% (95% CI: 5.0–75.4) at 7–90 days, and 35.9% (95% CI: −50.0–72.6) at >90 days after vaccination. VE of the JN.1-adapted sa-mRNA vaccine alone was 74.5% (95% CI: 31.3–90.6) at ≥7 days. JN.1-adapted mRNA vaccination, including the sa-mRNA vaccine, was estimated to provide protection against symptomatic SARS-CoV-2 infection among older adults in Japan.