Effect of varicella vaccine introduction on chickenpox incidence in Kingdom of Bahrain.

BACKGROUND: Chickenpox (varicella) is a highly contagious and self-limiting disease in children. However, complications and hospitalizations may occur. In Bahrain, the varicella vaccine was introduced into the routine childhood immunization schedule in May 2015. However, published data describing the epidemiological patterns of varicella postvaccine introduction are limited. PURPOSE: This study aimed to estimate varicella vaccine effectiveness (VE); investigate changes in varicella incidence and seasonal variation; compare age groups, healthcare utilization, and hospitalization; and identify predictors of vaccine uptake. METHODS: This retrospective national study included all clinically diagnosed varicella cases reported to the Public Health Directorate in Bahrain in 2010-2024. Annual and age-specific incidence rates and seasonal variations were calculated. Demographic characteristics, healthcare settings, and hospitalizations were compared by vaccination status. Varicella VE was estimated using a screening method. RESULTS: Of the 34,499 varicella surveillance records, 32,177 were confirmed as chickenpox cases (15,579 in 2010- 2013, 16,598 in 2014-2024). The annual incidence peaked at 251.8 per 100,000 in 2015 and declined to 28.5 per 100,000 by 2024 postvaccination (88.7% reduction). Seasonal peaks were more evident between 2014 and 2019 but became less pronounced thereafter. Most patients were male (57.4%), Bahraini nationals (69.1%), or unvaccinated (90.1%). The most affected age groups were children aged 5-9 years (36.1%) and 0-4 years (28.4%). More than 98% of the cases were managed in primary healthcare settings. Only 12 of the unvaccinated patients (0.1%) required hospitalization. The calculated VE ranged from 97.7% to 100%. Compared to children aged 0-4 years, all older age groups had significantly lower odds of vaccination (P<0.001). Similarly, non-Bahraini patients had a lower odd than Bahraini patients (odds ratio, 0.744; 95% confidence interval, 0.633- 0.875; P<0.001). CONCLUSION: The introduction of routine childhood varicella vaccination in Bahrain resulted in a substantial decline in the incidence of varicella, particularly among vaccine-eligible children. Severe diseases requiring hospitalization are rare and occur only in unvaccinated individuals. Age and nationality were independent predictors of vaccine uptake.