Use of anti-nucleocapsid antibody detection as a marker of SARS-CoV-2 infection in healthcare professionals: a longitudinal seroprevalence study at a university hospital in Catalonia, Spain

BACKGROUND: Healthcare workers (HCWs) are at increased risk of SARS-CoV-2 infection due to their continuous exposure during the COVID-19 pandemic. Serological detection of anti-nucleocapsid (anti-N) antibodies offers a valuable approach to estimate cumulative infection rates, including asymptomatic and undiagnosed cases missed by molecular testing. METHODS: We performed a longitudinal retrospective study involving HCWs at Althaia Xarxa Assistencial Universitària, Manresa (Spain), from June 2020 to November 2021. Participants underwent two rounds of serological testing with the Elecsys® Anti-SARS-CoV-2 assay (Roche) to quantify total anti-N antibodies. Concurrently, results from direct diagnostic tests (PCR or antigen detection) were collected. Seroprevalence, antibody concentrations, and infection rates—based on seroconversion and PCR/antigen positivity—were evaluated at both time points. RESULTS: Of the 758 HCWs included (84% female, median age 45 years), anti-N seroprevalence increased from 11.1% in June 2020 to 23.4% in November 2021. Median anti-N antibody levels declined significantly from 95.1 COI to 18.7 COI across the study period. The estimated infection rate between the two rounds was 12.1%. Serological testing identified a higher proportion of infections than direct diagnostic methods in the second period (23.4% vs. 17.9%). Three participants showed suspected reinfection, evidenced by rising antibody titers, and six participants seroreverted, all initially presenting low antibody levels. CONCLUSIONS: Anti-N serology effectively identifies past SARS-CoV-2 infections, particularly in asymptomatic HCWs undetected by PCR or antigen tests. Longitudinal monitoring reveals a marked decline in antibody concentrations over time. Incorporating anti-N serological testing alongside molecular diagnostics can enhance retrospective infection surveillance and aid in the detection of reinfection. TRIAL REGISTRATION: This study was retrospectively registered with the Institutional Ethics Committee of the Catalan Hospital Union (Unió Catalana d’Hospitals), reference number 40/21, on June 7, 2021.