Saraiva, Ana LeonorAfreixo, VeraMachado, AusendaGaio, Vânia2026-03-032026-03-032025-10http://hdl.handle.net/10400.18/11069The emergence of COVID-19 in 2019 led to the rapid development of vaccines and diagnostic tests. To assess antibody responses in healthcare workers (HCWs), a cohort study was conducted between 2021 and 2022 across three Portuguese hospitals. Antibody levels were measured at six time points: prevaccination, post-vaccination, and at 3, 6, and 12 months after the vaccination, as well as after the booster dose. Each hospital used a different assay: Abbott’s CMIA, Roche’s Elecsys® ECLIA, and Siemens’ ADVIA Centaur®, posing challenges for data comparability. The study aimed to harmonize serological data across these hospitals and to model antibody increases and decreases over time using linear regression. To ensure adequate conversion of antibody titers from different laboratory methods, quantile harmonization, and Deming regression were applied. After harmonization, three linear regressions were fitted: one for the increase between prevaccination and post-vaccination, another for the decrease between post-accination and 12 months after vaccination, and finally, one for the increase between 12 months after vaccination and after the booster dose. Models included variables such as prior infection, age, hospital, smoking status, contact with COVID-19 patients, and chronic conditions. In the phase-specific analysis, in addition to variations between hospitals in the regression of the last increase after the booster dose, it was observed that individuals over 50 years of age exhibited a superior immune response (811 550; IC 95%: 598 774, 1 024 327; p < 0.001). This higher percentage increase may be explained by initially lower levels, unlike younger individuals who had higher titers.engSARS-CoV-2IgG antibodyHealthcare WorkersHarmonizationVEBISDeterminantes da Saúde e da DoençaEstados de Saúde e de DoençaInfecções Respiratóriasconference object