Cellular and Humoral Immune Responses in Anti-N Seropositive and Seronegative Vaccinated Participants: Insights from the Bialystok PLUS Study

Background: In the post-pandemic era, SARS-CoV-2 immunity is increasingly shaped by vaccination and previous infection. Understanding the relationship between cellular and humoral immune responses is important for characterizing immunity in vaccinated populations. Methods: This study included 629 participants from the Bialystok PLUS cohort, stratified according to anti-N serostatus into anti-N-seronegative (n = 159) and anti-N-seropositive (n = 470) groups. Cellular immune response measurements were available for a subset of 246 participants. Humoral response was assessed using anti-S and anti-N antibodies, while cellular response (OC) was evaluated by interferon-gamma (IFN-γ) release following SARS-CoV-2 antigen stimulation. Group differences were assessed using the Mann–Whitney U test, and associations were evaluated using Spearman’s rank correlations with Benjamini–Hochberg correction. Multivariable linear regression was used to assess the association between anti-N serostatus and cellular response after adjustment for age and sex. Results: Anti-N-seropositive participants had significantly higher cellular responses than anti-N-seronegative participants (median 1752.3 vs. 771.1; p < 0.001). After adjustment for age and sex, anti-N seropositivity remained significantly associated with higher cellular response, corresponding to a 3.57-fold higher geometric mean of OC + 1 (95% CI: 1.90–6.73; p < 0.001). Anti-S antibody levels showed moderate positive correlations with cellular response in both anti-N-seronegative and anti-N-seropositive participants (ρ = 0.45 and ρ = 0.37, respectively), and the association remained significant after adjustment for age, sex, and anti-N serostatus. No significant associations were observed between cellular response and anti-N antibody levels or routine laboratory parameters after correction for multiple comparisons. Conclusions: Anti-N seropositivity was associated with higher SARS-CoV-2-specific cellular immune response among vaccinated participants, and this association remained significant after adjustment for age and sex. The moderate association between anti-S antibody levels and cellular response suggests that humoral and cellular measures provide related but complementary information on SARS-CoV-2-specific immunity. Routine laboratory parameters were not significantly associated with cellular immune response in this cohort.