Persistent Varicella Transmission in a High-Coverage Single-Dose Vaccination Program: Costa Rica, 2015–2024

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Abstract
Background: Costa Rica introduced universal one-dose varicella vaccination in 2007. Although the disease burden subsequently declined, varicella cases and hospitalizations continue to be reported. We characterized varicella epidemiology and healthcare burden in Costa Rica using the most recent nationally available data. Methods: We conducted a nationwide retrospective descriptive study using aggregated surveillance and ad-ministrative data from the Ministry of Health and the Costa Rican Social Security Fund. Outcomes included notified incidence (2015–2022), outpatient and emergency department visits (2019–2024), hospitalizations (2015–2024), in-hospital deaths associated with varicella (2018–2020), reported institutional outbreaks, and first-dose vaccination coverage (2018–2024). Results: Between 2015 and 2022, incidence declined by 84% in males and 83% in females, although it increased until 2019 and declined markedly during the COVID-19 pandemic. Infants and children aged 1–9 years accounted for the largest proportion of reported cases. Among 354 hospitalizations, adults aged 20–44 years accounted for 30.5%. Comparing 2015–2018 with 2021–2024, hospitalizations declined by approximately 56% among chil-dren aged < 4 years and increased by approximately 200% among adults aged 20–44 years. Three varicel-la-associated in-hospital deaths, all in adults, and one institutional outbreak were identified. First-dose coverage averaged approximately 92% during 2018–2022 and exceeded 95% in 2023–2024. Conclusions: Despite substantial reductions in reported incidence, varicella circulation and healthcare burden persist, with hospitalizations increasingly concentrated among adults. These findings support sustaining high first-dose coverage, strengthening surveillance, and evaluating the potential contribution of a two-dose vac-cination strategy.
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