Eurosurveillance - Current Issue -Volume 31, Issue 9, 05/Mar/2026

Global resurgence of measles highlights the need for countries to identify and address immunity gaps. This is challenging given antibody waning and, in Ireland, the absence of robust national vaccine coverage data, population changes due to migration and outdated population-level seroprevalence data from the last national serosurvey (2003).

We aimed to determine the seroprevalence of measles IgG in children aged 3–17 years in Ireland.

Convenience sampling of anonymised residual serum samples from four hospital laboratories across four of six health regions was conducted between 1 February and 19 June 2024. Samples were tested for measles IgG antibodies using a commercial chemiluminescence immunoassay. Seropositivity was adjusted for test sensitivity and specificity and was calculated by sex, age and location.

In total 2,509 of 2,924 samples were seropositive and 415 were seronegative indicating measles IgG seroprevalence of 90.3% (95% confidence interval (CI): 89.2–91.4), with no significant difference by sex. Children aged 3–5 years (94.9%; 95% CI: 92.4–96.6) and 6–9 years (94.2%; 95% CI: 91.7–95.9) had significantly higher seropositivity when compared with children aged 10–13 years (89.1%; 95% CI: 86.6–91.3) and 14–17 years (87.6%; 95% CI: 85.5–89.4).

Our findings suggest close to adequate protection against measles among children 3–9 years but suboptimal (< 95%) protection among children aged 10–17 years. This immunity gap is not reflected in measles vaccine coverage data, highlighting the utility of seroprevalence data to enhance knowledge of clinical protection at population level and to inform vaccination strategies.

In autumn 2023, Spain recommended nirsevimab to all infants born after 1 April 2023, as catch-up or at-birth immunisation.

We estimated effectiveness of a single nirsevimab dose against respiratory syncytial virus (RSV) hospitalisations throughout two seasons in healthy term-born infants.

Cases were children born 1 April 2023 through 31 March 2024 after 35 gestation weeks without major comorbidities and hospitalised for RSV infection between 2023 immunisation campaign onset and 31 March 2025. We selected four healthy population-density controls per case, matched by province and birth date. Using target trial emulation, causal per-protocol effectiveness was estimated for catch-up (within 30 days of 2023 campaign onset) and at-birth immunisation (within 14 days of life) through cloning, censoring and inverse-probability-weighted conditional logistic regression.

We included 235/905 cases/controls for catch-up and 334/1,292 cases/controls for at-birth immunisation (first season), and 188/713 cases/controls for catch-up and 328/1,269 cases/controls for at-birth immunisation (second season). Two-season effectiveness was 64% (95% confidence interval (CI): 52–72) and 67% (95% CI: 59–74) for catch-up and at-birth immunisation, respectively, compared with 78% (95% CI: 70–84) and 84% (95% CI: 79–88) during first season and −8% (95% CI: −88 to 38) and 20% (95% CI: −21 to 46) during second season.

Nirsevimab was an effective long-term population-level intervention, decreasing RSV hospitalisations by two-thirds during the first two seasons of life. Effectiveness during second season was low or null, although it may be underestimated due to unavoidable survivor bias. The RSV hospitalisation rate among immunised children did not rebound in the second season.