Real-world immune dynamics following COVID-19 vaccination and breakthrough infection: a paired-sample study in Zhejiang Province, China.

Introduction: As SARS-CoV-2 variants continue to spread, breakthrough infections (BTI) provide real-world insights into the durability and heterogeneity of COVID-19 vaccine-induced immunity. This study aimed to characterize the immune response dynamics and reconstruction characteristics after BTI in participants with different vaccination regimens in Zhejiang Province, China. Methods: A total of 370 participants were enrolled from a stratified random sample of permanent residents. Participants were either unvaccinated or had received one of the following vaccine regimens: one dose of adenoviral vector vaccine, two or three doses of inactivated vaccine, or, three doses of recombinant protein subunit vaccine. Paired serum samples were collected after vaccination and after BTI. RBD-IgG levels, pseudovirus neutralizing activity, and eight cytokines were systematically quantified. Correlation analyses and post-infection lower-quartile subgroup (T2-LQ) analyses were performed to assess immune heterogeneity. Results: All vaccine groups exhibited robust seroconversion, with post-vaccination RBD-IgG positivity exceeding 90% for both inactivated and recombinant vaccines. Following BTI, antibody levels and neutralizing activity increased significantly across all vaccine groups, consistent with the boosting effect of hybrid immunity. Cytokine profiling analyses revealed minimal differences between vaccine groups post-infection, with immune dynamics primarily driven by longitudinal changes. Notably, IL-13 levels declined consistently across all groups, accompanied by modest changes in the overall cytokine profile. Correlation analyses did not identify a stable concordance between antibodies and cytokines. However, variant-specific RBD-IgG measurements were highly correlated at both time points. Additionally, a post-infection lower-quartile subgroup (T2-LQ) was identified. Among BTI participants, baseline WT-specific RBD-IgG levels were comparable between T2-LQ and non-LQ individuals, suggesting that the low-responder phenotype is not simply attributable to pre-infection antibody levels. Conclusion: This paired-sample, population-based study suggests that BTI may further enhance humoral responses and be accompanied by changes in inflammatory/immunoregulatory markers on top of prior vaccine-induced immunity, while reducing some overall immune differences across vaccine platforms. The identification of the T2-LQ subgroup highlights persistent heterogeneity in post-infection immune responses and suggests the potential value of continued immune monitoring in the post-pandemic era.