Abstract / Summary
Understanding longitudinal humoral immune responses in SARS-CoV-2 convalescent populations is important for characterizing immune persistence and informing follow-up surveillance. However, evidence from large longitudinal cohorts spanning the full age range remains limited. This longitudinal cohort study was conducted in Fuzhou, China, from March to September 2023. A total of 2,482 convalescent individuals were assessed at baseline, Month 3, and Month 6. Serum IgG concentrations were measured using a chemiluminescence immunoassay. Linear mixed-effects models with Yeo–Johnson transformation were used to evaluate longitudinal IgG changes and associated factors, with multiple imputation for missing follow-up IgG measurements. IgG trajectories differed across age groups. IgG levels generally declined over follow-up, although the timing and magnitude of change varied by age. Participants aged 0–18 years had higher baseline IgG levels and a more pronounced overall decline, whereas those aged ≥ 65 years showed no significant change from baseline to Month 3 followed by a subsequent decrease. Sex was not associated with IgG levels. Asymptomatic status was associated with lower IgG than symptomatic infection with recovery within ≤ 1 week, while recent vaccination was associated with higher IgG and smoking history with lower IgG. Inter-district heterogeneity in IgG levels was also observed. SARS-CoV-2 IgG dynamics during follow-up showed clear age-related heterogeneity in this convalescent cohort. Older adults exhibited a relatively stable early phase followed by later decline, whereas younger groups showed earlier decreases. Recent vaccination, symptom status, and smoking history were associated with IgG levels, while observed inter-district differences warrant further investigation. These findings support further age-stratified longitudinal surveillance research.