Causes of a Low Measles Seroprevalence among Young Healthcare Workers in Korea

Background: Sporadic measles outbreaks have continued to occur in Korea, mainly in adults in their 20s and 30s, most notably in 2014 and 2019. We here evaluated the possible causes of a low seroprevalance of measles by testing young healthcare workers (HCWs).Materials and Methods: This study was conducted in a 2,743-bed tertiary-care hospital in Seoul between 2020 and 2021. We performed a measles antibody test (chemiluminescence immunoassay), measured the IgM/IgG index ratio, and conducted an avidity test at 1-month after Measles-Mumps-Rubella (MMR) vaccination in HCWs who had been seronegative for measles. Measles vaccination histories were obtained from the national vaccine registry.Results: Of the 3,173 HCWs newly employed in our hospital during the study period, 54 with a negative measles IgG at commencement were enrolled. Thirty six (67%) of these subjects were female, and the median age was 25 years (interquartile range [IQR]: 24 -27). Fourty nine (91%) showed seroconversion at 1 month after the first vaccination. Of these individuals, 38 received both measles IgM and IgG test, and all had an IgM/IgG index <1. Of the 49 seroconverters, all HCWs showed a high avidity index. According to the national immunization registry, 45 (83%) received at least 2 doses of an MMR vaccination.Conclusion: Secondary vaccine failure may underlie vaccine failure in young Korean adults. HCWs born after 1985 with a negative measles antibody may need only a single dose booster vaccination rather than a 2-dose vaccination regimen.