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

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. 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. 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. 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. 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.