Nucleocapsid-directed antibody testing is unsuitable to estimate hybrid immunity against SARS-CoV-2, a longitudinal cross-border study in the Meuse-Rhine Euroregion

Abstract

Introduction

Understanding immunity from previous natural SARS-CoV-2 infection is important for booster vaccination strategies. A longitudinal study conducted in 2021 within the Meuse-Rhine Euroregion, bordering the Netherlands, Belgium, and Germany, aimed to assess seroprevalence of spike-directed (anti-S) and nucleocapsid-directed (anti-N) antibodies to demonstrate immunity as a result of both vaccination and natural infection (hybrid immunity), and to evaluate the dynamics of the anti-N response.

Materials and methods

Questionnaires and self-finger-prick blood samples from 3110 participants were collected at two time points: weeks 22–29 (June–July, round 1) and weeks 40–45 (October–November, round 2) of 2021. Individuals with anti-S antibodies were additionally tested for anti-N antibodies.

Results

In total, 4366 samples tested positive for anti-S; n = 1291 for round 1 and n = 3075 for round 2. Of these, 10.1 % of Dutch (32/316), 3.1 % of Belgian (9/294), and 2.8 % of German participants (13/466) were anti-N positive in round 1 (p < 0.001). In round 2, this was 4.6 % (69/1510), 3.3 % (20/607), and 1.5 % (14/912), respectively (p < 0.001). In 45.1 % (23/51) of anti-N positive participants in round 1, the result reversed to negative in round 2. In 42.1 % (16/38) of anti-N positive participants with a self-reported positive PCR result, anti-N reversed to negative in round 2.

Conclusion

Variations in anti-N seroprevalence across EMR countries may reflect differences in vaccination campaign enrollment. Over 40 % of participants experienced seroreversion of anti-N within six months, indicating anti-N testing is unsuitable for diagnosing past infection or estimating hybrid immunity within a population. However, anti-N testing may be used as a proxy for increased circulation of SARS-CoV-2 in a population.