Impacto da vacinação contra a COVID-19 em Guaramiranga, Ceará, 2021-2022: uma análise integrada da efetividade vacinal, desfechos clínicos e comportamentos preventivos

Abstract

This thesis aimed to evaluate, among residents of Guaramiranga, Ceará, Brazil, from 2021 to 2022, the impact of COVID-19 vaccination through an integrated approach that estimated seroprevalence and the duration of the post-vaccination humoral response, characterized severe clinical outcomes, and analyzed preventive behaviors. Guaramiranga was the first municipality to achieve 100% vaccination coverage of its adult population (4,002 residents) in 2021, predominantly with CoronaVac (3,328 doses administered), complemented by AstraZeneca (1,685) and Pfizer-BioNTech (174). Results were organized into three chapters presented as scientific articles. Chapter 1 (Article 1) addresses seroprevalence and post-vaccination humoral response. This analytical cross-sectional study investigated the seroprevalence of anti-SARSCoV-2 neutralizing IgG and associated factors among 1,714 residents who received two doses of CoronaVac. Seroprevalence was 94.1% between 91 and 180 days after the second dose, declining to 79.4% after six months (p < 0.001), indicating waning humoral immunity. RT-PCR positivity was 15.3% (95% CI: 12.8–18.2), increasing to 29.0% after 270 days since the second dose (p = 0.0006), suggesting increasing susceptibility to infection over time. Lower seropositivity was associated with longer time since the second dose (RR = 0.9987; p < 0.001), older age (RR = 0.9924; p < 0.001), and male sex (RR = 0.9176; p = 0.018). During follow-up, 116 symptomatic cases and two severe cases were recorded in 2022 (0.117%) among the vaccinated population. Chapter 2 (Article 2), conducted as a case series, characterized severe clinical outcomes of COVID-19 among hospitalized residents and deaths occurring from March 2020 to December 2022. Nine hospitalizations were identified, seven of which resulted in death. Among these nine cases, two were male, the median age was 87 years, five had an incomplete vaccination schedule, and hypertension and diabetes were the most common comorbidities. The main symptoms at admission were fever (4), cough (4), and dyspnea (5). Regarding treatment, 4/9 received anticoagulants (enoxaparin), corticosteroids, and supplemental oxygen; 2/9 received oseltamivir; and 3/9 received empirical antibiotic therapy for pneumonia. The mean interval between the last vaccine dose and hospitalization was 128 days (range: 28–326), consistent with a potential decline in vaccine-derived protection over time. Chapter 3 (Article 3), also cross-sectional, evaluated preventive behaviors among 2,830 vaccinated adults between September 2021 and December 2022, indicating sustained protective practices: 65.3% reported frequent handwashing and 63.8% reported mask use when leaving home, and 59.7% reported maintaining preventive practices after vaccination. Higher adherence to preventive behaviors was observed among women, younger adults, and individuals with chronic diseases or greater knowledge about covid-19 (p < 0.05), whereas lower educational attainment was associated with greater isolation and distancing (p < 0.05), reinforcing the role of social determinants and risk perception in sustaining these measures. Overall, the integrated findings indicate that realworld CoronaVac vaccination, combined with the maintenance of preventive behaviors among higher-risk groups, contributed substantially to reducing severe cases and deaths, even under conditions of high viral transmission and limited health-system capacity. The convergence of humoral immune response (seroprevalence and temporal antibody decline), protection against severe outcomes (low hospitalization frequency and severe-case burden), and preventive behaviors (persistence of hygiene measures and isolation/distancing practices) provides a comprehensive model of population-level protection in small municipalities, with important implications for public health policies in similar contexts, particularly regarding booster-dose strategies, health education tailored to different social groups, and ongoing epidemiological surveillance. In the current context of sustained SARS-CoV-2 circulation and the need to preserve health-system response capacity, this thesis supports the view that booster recommendations should account for time since the last dose, age, and vulnerability, and that risk communication and nonpharmacological interventions can be strategically intensified during higher-risk periods, such as transmission surges.