Severe Varicella in Children Under Five Years of Age in the Context of Herd Immunity, Vaccine Hesitancy, and the COVID-19 Pandemic in Brazil, 2008-2023

Abstract

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.