Trajectories and determinants of respiratory syncytial virus prefusion F-specific IgG in older adults

BACKGROUND: Respiratory syncytial virus (RSV) causes substantial morbidity in older adults. However, population-level longitudinal patterns and variability of naturally acquired antibody responses remain insufficiently quantified. We aimed to quantify population-level trajectories and variance components of RSV prefusion F (preF)-specific IgG in older adults. METHODS: We conducted a prospective cohort study of 132 adults aged ≥60 years using residual serum samples, with repeated measurements over one year (666 observations). RSV preF-specific IgG levels were measured by enzyme-linked immunosorbent assay. Population-level trends were estimated using a linear mixed-effects model with a random intercept, adjusting for key covariates. Exploratory analyses were summarized descriptively to characterize variability. RESULTS: At the population level, antibody levels showed no significant temporal change (β = 0.004, SE = 0.003; p = 0.152). Between- and within-individual variability were comparable (SD ≈ 0.247 vs 0.248). Individual slopes were centered near zero (median ≈ 0) with a wide, approximately symmetric distribution. A subset (~31%) exhibited higher variability. No chronic condition was associated with antibody levels after correction for multiple comparisons. CONCLUSIONS: RSV preF-specific IgG levels in older adults were stable at the population level over one year, with substantial within- and between-individual variability. These findings reflect population-average trends and variance components estimated from a random-intercept model and should not be interpreted as evidence of individual-level trajectories or underlying biological mechanisms. CLINICAL TRIAL NUMBER: Not applicable.