435. Investigation of the cause of low measles seroprevalence among young healthcare workers in South Korea


 
 
 After a large measles outbreak in 2000∼2001, national measles vaccination program was implemented in Korea, and more than 95% of people born after 1985 have been vaccinated with 2-dose MMR. But sporadic outbreaks have continued to occur mainly in adults in their 20’s and 30’s in 2014 and 2019. Although low prevalence in young adults have been reported, there are limited data regarding the cause of low seropositivity (positive vaccine failure vs. secondary vaccine failure). We aimed to evaluate the cause of low seroprevalence of measles.
 
 
 
 This study was performed in a 2743-bed tertiary-care hospital in South Korea. At 1-month after MMR vaccination in healthcare workers (HCWs) who were seronegative for measles, we performed measles antibody test(enzyme immunoassay), IgM/IgG index ratio, and avidity test.
 
 
 
 Of the 1,698 new HCWs in 2020 and, 1,686 in 2021, 54 HCWs with negative measles IgG at their entering at our hospital were enrolled in this study. Of these, 36(67%) were female, and the median age was 25 (IQR, 24-27) years. Of 43 HCWs who measured IgM/IgG index 1 month after MMR vaccination, 38 HCWs had IgM/IgG index of less than 1 and remaining 5 had negative IgG results (seroconversion rate of 88%). All 54 healthcare workers who performed avidity test showed high avidity index Measles vaccination record in KCDA showed that 45 had documented vaccination record during their childhood (of these, 11 received MMR vaccine again during their adult age) and remaining 9 had no documented vaccination record during their childhood. Of these 9 healthcare workers without record in their childhood, 3 had a history of vaccination at adult. Analyzing 48 healthcare workers with vaccination record, they received MMR vaccination median 18 years (IQR, 7-20) before. Therefore, at least 48 received at least 2 dose MMR vaccination, and 11 received at least 3 dose MMR vaccination.
 
 
 
 Secondary vaccine failure may be the reason for vaccine failure in young adults in Korea. Therefore, HCWs with negative measles antibody who were born after 1985 may receive 1 dose booster vaccination rather than 2 dose vaccination.
 
 
 
 All Authors: No reported disclosures.
