Cross-sectional seroprevalence study of measles antibodies among children to identify gaps in population immunity, Ireland, 2024.

BACKGROUNDGlobal 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waning and, in Ireland, the absence of robust national vaccine coverage data, population changes due to migration and outdated population-level seroprevalence data from the last national serosurvey (2003).AIMWe aimed to determine the seroprevalence of measles IgG in children aged 3-17 years in Ireland.METHODSConvenience 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.RESULTSIn 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).CONCLUSIONOur 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.