Mucosal and Serum Neutralization Immune Responses Elicited by COVID-19 mRNA Vaccination in Vaccinated and Breakthrough-Infection Individuals: A Longitudinal Study from Louisville Cohort

<b>Background/Objectives</b>: The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus type-2 (SARS-CoV-2), has resulted in 777 million cases worldwide. Various vaccines have been approved to control the spread of COVID-19, with mRNA vaccines (Pfizer and Moderna) being widely used in the USA. We conducted a prospective longitudinal study to analyze the immune response elicited by two/three and four doses of monovalent mRNA vaccines in both vaccinated individuals and those who experienced breakthrough infections. Participants were stratified into different age groups: 18-40, 41-60, and over 60 years. <b>Methods</b>: We assessed cross-variant neutralization responses in two cohorts-Cohort I: n = 167 (serum), Cohort II: n = 92 (serum and nasal swab) samples-using infectious virus microneutralization assay (MN) and antibody (IgG or IgA) binding ELISA titers to the spike protein receptor binding domain (RBD). Samples were collected from the Louisville Metro-Jefferson County Co-Immunity Project, a federally funded, population-based study for the surveillance of SARS-CoV-2 in Jefferson County, Kentucky during 2020-2022, involving both health care workers and a local community. <b>Results</b>: Individuals who received two doses of the mRNA vaccine exhibited reduced neutralization against Beta, Delta, and Omicron BA.1 variants compared to wildtype Wuhan, with further decline observed six months post-booster vaccination. However, individuals who experienced natural COVID-19 infection (breakthrough) after receiving two vaccine doses showed enhanced neutralization and antibody responses, particularly against Omicron BA.1. Following the 3rd dose, antibodies and neutralization responses were restored. Among triple-vaccinated individuals, reduced neutralization was observed against Omicron variants BA.1, BA.5, and BA.2 compared to Wuhan. Neutralization responses were better against BA.2 variant compared to BA.1 and BA.5. However, individuals who received three doses of vaccine and experienced a breakthrough infection (n = 45) elicited significantly higher neutralizing antibodies responses against all Omicron subvariants compared to vaccinated individuals. Interestingly, nasal swab samples collected from volunteers with breakthrough infection showed significantly elevated spike-reactive mucosal IgA antibodies and enhanced cross neutralization against BA.1, BA.2, and BA.5 compared to individuals who received only three vaccine doses. <b>Conclusions</b>: mRNA vaccination elicits a strong systemic immune response by boosting serum neutralizing antibodies (NAb), although this protection wanes over time, allowing new variants to escape neutralization. Breakthrough individuals have extra enrichment in nasal NAb offering protection against emerging variants. This longitudinal immune profiling underscores the strengthening of pandemic preparedness and supports the development of durable mucosal vaccines against respiratory infectious disease.