Seroprevalence, immune response dynamics, and incidence of COVID-19 after CoronaVac vaccination in a fully vaccinated small municipality in Northeast Brazil: a population-based surveillance study

CoronaVac was the first COVID-19 vaccine deployed at scale in Brazil, yet evidence on its real-world impact and the durability of immune responses in small municipalities remains limited. Understanding vaccine performance in diverse settings is essential for informing public health strategies. This study aimed to evaluate the seroprevalence and durability of the immune response and investigate the real-world impact of the CoronaVac vaccine in adults from a small municipality in Northeast Brazil. This is an analytical, observational, population-based study comprising a cross-sectional seroepidemiological survey and a retrospective cohort analysis conducted in Guaramiranga, Ceará, between September 2021 and August 2022. A total of 1,714 individuals who had received two doses of CoronaVac were included. Serological survey (anti-SARS-CoV-2 IgG), molecular testing (RT‒PCR), and linkage of official vaccinations and notification records were performed. The analyses included Fisher’s Exact Test for categorical comparisons and Poisson regression with robust variance to estimate prevalence ratios. The seroprevalence of neutralizing IgG antibodies was 83.1% (95% CI: 79.7–86.0). Seropositivity showed a significant temporal decline, decreasing from 94.3% (≤ 180 days post-vaccination) to 79.4% (181–270 days post-vaccination) (p = 0.0012). RT‒PCR positivity was 15.3% (95% CI: 12.8–18.2%), increasing to 29.0% 270 days after the second dose (p = 0.0006). In multivariable Poisson regression, time since D2 (PR = 0.9987; p < 0.001), age (PR = 0.9924; p < 0.001), and male sex (PR = 0.9176; p = 0.018) were associated with lower seropositivity. A total of 116 symptomatic cases and 2 severe cases (0.117%) were confirmed, corresponding to an observed incidence of severe disease of 117 per 100,000 person-years (95% CI: 14–422). This population-based surveillance study demonstrates that universal CoronaVac vaccination was followed by a robust initial humoral immune response, which subsequently declined over time. The observed low incidence of severe disease in this fully vaccinated population is consistent with protection against severe outcomes. These findings reinforce the need for booster doses and ongoing epidemiological surveillance in small municipalities. Not applicable.