Abstract
BACKGROUND: Obesity significantly increases the risk of severe COVID-19 and mortality. Concerns have emerged regarding the efficacy of COVID-19 vaccines in obese populations due to potential alterations in immune responses. METHODS: Between March 2021 and March 2022, 5,071 participants with comorbidities received CoronaVac administered in two doses 28 days (± 7 days) apart and a single subsequent booster dose of ChAdOx1 with an interval of 180 days (± 30 days). Body composition was analyzed using a tetrapolar bioimpedance measurement scale. Anti-RBD IgG was dosed at baseline (D0) and on days 28, 90, 180, 270, and 360. RESULTS: 1,181 participants with obesity (body fat percentage > P50), non-immune at baseline, showed significantly lower antibody titers (IgG anti-RBD) until D180 than 576 non-obese individuals. However, there was no difference in the frequency of COVID-19, COVID-19-related hospitalization or death. After the booster, no significant changes were seen. Patients with very excessive body fat (body fat percentage > P90) showed even slower seroconversion, with increased COVID-19 after D180. Similar results were observed using the routine body mass index (BMI); however, waning immunity (serology) was observed in individuals classified as having very excessive body fat only when bioimpedance was used. CONCLUSION: Participants with obesity demonstrated weaker and slower seroconversion, which worsened in higher body fat mass. Bioimpedance revealed quicker waning immunity, emphasizing its importance over BMI in assessing vaccine response. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12879-025-12295-2.