Clinical Scorecard: Assessment of anti-nucleocapsid antibody presence as an indicator of SARS-CoV-2 infection among healthcare workers: a longitudinal seroprevalence investigation at a university hospital in Catalonia, Spain

At a Glance

Key Highlights

• Anti-N antibody detection distinguishes natural infection from vaccination response since vaccines target the spike (S) protein only.
• Longitudinal seroprevalence study conducted in two phases (June–July 2020 and October–November 2021) among vaccinated healthcare workers.
• Reinfection defined by increase in anti-N antibody titer between study phases; PCR and antigen tests used symptomatically or for contact screening.

Guideline-Based Recommendations

Diagnosis

• Use anti-N antibody serology to confirm SARS-CoV-2 infection in vaccinated individuals.
• Employ PCR testing with amplification of at least two target markers for definitive diagnosis.
• Use Rapid Antigen Tests (RAT) for symptomatic or contact-screened healthcare workers per occupational health protocols.

Management

• Vaccinate healthcare workers with two doses of Comirnaty® (Pfizer/BioNTech) as per local program.
• Implement infection control protocols including PCR Ct value thresholds (Ct >40 negative, Ct ≤30 for isolation clearance).

Monitoring & Follow-up

• Perform longitudinal serological testing for anti-N antibodies to monitor infection rates and reinfections.
• Conduct PCR/RAT testing symptomatically or after exposure according to hospital occupational health guidelines.

Risks

• Healthcare workers are at high risk of SARS-CoV-2 exposure and potential asymptomatic transmission.
• Serological testing alone cannot exclude infection; combined diagnostic approaches recommended.

Patient & Prescribing Data

Healthcare workers vaccinated with two doses of Comirnaty®

Vaccination induces anti-S antibodies but not anti-N; anti-N serology is essential to identify breakthrough infections or reinfections.

Clinical Best Practices

• Use anti-N antibody testing to differentiate natural infection from vaccine-induced immunity in vaccinated populations.
• Follow strict occupational health protocols for PCR and antigen testing based on symptoms or exposure.
• Interpret serological results using manufacturer cutoffs: COI >1.1 positive, <0.9 negative, 0.9–1.1 grey zone.
• Consider longitudinal antibody titer changes to identify possible reinfections.

References

• Roche Elecsys Anti-SARS-CoV-2 assay
• ThermoFisher TaqPath COVID-19 Combo Kit
• Abbott Panbio COVID-19 Ag Rapid Test Device