Evaluation of Ireland’s Varicella Surveillance Systems

Varicella (chickenpox) is a highly infectious disease causing severe complications, particularly in vulnerable groups. In October 2024, varicella vaccination was introduced into Ireland’s national childhood immunisation programme, creating a need for robust surveillance to monitor disease trends and vaccine impact. This evaluation assessed whether Ireland’s varicella surveillance systems are fit for purpose in capturing disease epidemiology, severity, and changes following vaccine introduction.

A mixed-methods evaluation was conducted for 2022–2024 per ECDC, WHO and CDC guidance. Quantitative analysis drew on national statutory surveillance, hospital discharge, sentinel GP, and mortality data. Aggregate comparisons assessed completeness, representativeness and validity. A stakeholder survey evaluated usefulness, simplicity and acceptability. Two fundamental accuracy problems were identified. First, most hospitalised cases were not captured: 474 statutory notifications versus 1,158 hospital discharges (40.9%), with capture poorest in children aged 1–4 years (19.5%) and 5–14 years (22.2%). Second, older age groups were over-represented, consistent with misclassification of herpes zoster. Core clinical variables and vaccination status were incompletely recorded, and marked regional variation was observed. Closer alignment was seen for severe neurological complications. Ireland’s varicella surveillance system supports statutory functions but does not currently provide a reliable estimate of hospitalised disease burden, limiting capacity to evaluate vaccine impact. Five recommendations were made, covering: epidemiological analysis to quantify under-ascertainment and misclassification (including capture-recapture study); completeness and case classification; proportionate implementation of a new national surveillance platform; notification scope; and sustaining a multi-source architecture. Addressing these gaps is essential for effective post-vaccine monitoring.

Key messages

• Ireland’s varicella surveillance underestimates hospitalised cases, limiting its ability to measure true disease burden and vaccine impact without system strengthening.

• Improving data completeness, case classification and linkage with immunisation data is essential to ensure surveillance can support post-vaccine monitoring and policy decisions.