Shahrekord University of Medical Sciences Epidemiology and Health System Journal 2980-7891 13 2 2026 07 30 Factors Associated With Post-Vaccination COVID-19 Infection Leading to Hospitalization in Chaharmahal and Bakhtiari Province: A Case-Control Study 56 77 10.34172/ehsj.26698 EN Kosar Farhadi Babadi https://orcid.org/0000-0002-3607-2298 Seyed Saeed Hashemi Nazari https://orcid.org/0000-0002-0883-3408 Amir Kavousi https://orcid.org/0000-0003-3922-0564 Ali Ahmadi https://orcid.org/0000-0002-1588-2136 Mohadeseh Karimian https://orcid.org/0000-0002-8462-9118 Journal Article 10.34172/ehsj.26698 2026 02 15 2026 06 14 Introduction: COVID-19 vaccination and its associated complications have been a controversial issue worldwide. This study aimed to identify the determinants of infection leading to hospitalization following COVID-19 vaccination in Chaharmahal and Bakhtiari Province. Methods: In this case-control study, after screening records from 47,001 individuals, 260 hospitalized vaccinated individuals were selected as cases, while 130 vaccinated outpatients and 130 healthy individuals were included as controls (n = 520). Data were analyzed by Stata 17 using descriptive and inferential statistics, including univariate and multivariable analyses and ordinal logistic regression models. Results: Multivariable analyses revealed older age (> 55 years, OR = 11.08, P < 0.001), female gender (OR = 0.45, P = 0.002), prior infection (OR = 292.7, P = 0.001), and non-healthcare occupations (OR = 2.07, P = 0.036) as significant risk factors for hospitalization, while being male (OR = 0.45, P = 0.002) and a healthcare worker (OR = 0.22, P = 0.006) were protective. Moreover, the Sinopharm (OR = 2.40, P = 0.012) and other vaccines (OR = 2.54, P = 0.036) were associated with higher hospitalization risk compared to AstraZeneca. Additionally, the protective effect of vaccination and underlying disease decreased over time (underlying disease HR = 0.99, P = 0.014; homologous booster HR = 0.99, P < 0.001; heterologous booster HR = 0.99, P = 0.025). Conclusion: Age, gender, comorbidities, recent hospitalization, and high-risk occupation influenced COVID-19 hospitalization, while prior infection and vaccination provided temporary protection, underscoring the need for targeted, time-sensitive interventions.