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

Background: Costa Rica introduced universal one-dose varicella vaccination in 2007 using a one-dose schedule administered at 15 months of age through the Costa Rican Social Security Fund (CCSS). A previous national study described epidemiological changes through 2014 by comparing pre- and post-vaccination periods; however, contemporary data on subsequent patterns and residual disease burden are limited. 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 administrative data. Outcomes included incidence of reported varicella cases (2015-2022), outpatient and emergency department visits (2019-2024), hospitalizations (2015-2024), in-hospital deaths with a recorded varicella diagnosis (2018-2020), institutional outbreaks, and first-dose vaccination coverage (2018-2024). Results: From 2015 to 2022, reported incidence declined by 84% in males and 83% in females, after increasing through 2019 and declining markedly during the COVID-19 pandemic. 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%. Hospitalizations among children aged ≤4 years decreased by approximately 56% between 2015-2018 and 2021-2024, while annual hospitalizations among adults aged 20-44 years increased from 7 in 2015 to 21 in 2024, representing a 200% increase over the study period. Three deaths occurred during hospitalizations with a varicella diagnosis, all among adults, and one institutional outbreak was identified. First-dose vaccination coverage averaged approximately 94% during 2018-2022 and remained high thereafter. Conclusions: Reported varicella incidence declined substantially during the study period, but circulation and healthcare burden persisted, with hospitalizations increasingly concentrated among adults. Because our analysis lacks a pre-vaccination comparator, these trends cannot be causally attributed to vaccination based on our data alone. The findings support maintaining high first-dose coverage and strengthening surveillance. Further information on individual vaccination history and population susceptibility is needed to evaluate the potential contribution of a two-dose vaccination strategy in Costa Rica.