Tehran University of Medical Sciences Journal of Family and Reproductive Health 1735-8949 20 2 2026 09 13 126 133 EN Maria Nascimento Department of General and Specialized Surgery, Postgraduate Maternal Child Health Program, Faculty of Medicine, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil Alcenir Valente Junior Faculty of Medicine, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil Beatriz de Falcão Faculty of Medicine, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil Marina Silva Faculty of Medicine, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil Mauricio Leão Postgraduate Maternal Child Health Program, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil 2026 02 13 2026 08 05 Objective: Varicella (chickenpox) is a highly contagious disease, and its vaccine was incorporated into the Brazilian National Immunization Program in 2013. Objectives: (i) To analyze trends in hospitalization rates for severe chickenpox by age; (ii) To assess the impact of the interventions “anti-vaccine movement/vaccine hesitancy” and “COVID-19 pandemic” on hospitalization rates. Materials and methods: Classical Time Series (CTS) and Interrupted Time Series (ITS) methods were applied to estimate the impact of the anti-vaccine movement/vaccine hesitancy (intervention-1) that occurred in 2016 and the COVID-19 pandemic (intervention-2) that began in 2020 on hospitalization rates among children under 5 years old, stratified by groups based on vaccine-ineligible (< 1 year old) and vaccine-eligible (1-4 years old) children. Results: CTS results supported the potential for herd immunity effects, showing an annual decline of -17% and -14% in hospitalization rates for vaccine-eligible (1-4 years) and vaccine-ineligible (< 1 year) groups, respectively. The ITS model for vaccine-ineligible children showed an upward trend (coefficient 0.35) after intervention-1(isolate) (p=0.001), as well as after intervention-1 (coefficient 0.42; p 0.011) adjusting for intervention-2 (p=0.135). For vaccine-eligible children, the ITS model revealed an upward trend (coefficient 0.14) only after intervention-1 (isolate) (p=0.006).   Conclusion: The decline in varicella hospitalizations indicates a herd immunity effect among vaccine-ineligible children. ITS analysis reveals that the anti-vaccine movement/vaccine hesitancy contributed to increased hospitalizations in both vaccine-eligible (1-4 years) and vaccine-ineligible (<1 year) groups, while the COVID-19 pandemic had no discernible impact. https://jfrh.tums.ac.ir/index.php/jfrh/article/view/3670 https://jfrh.tums.ac.ir/index.php/jfrh/article/download/3670/763