Control of a nosocomial measles outbreak among previously vaccinated adults in a population with high vaccine coverage: Korea, 2019

We describe a measles outbreak among previously vaccinated healthcare workers (HCWs) and inpatients and the control measures implemented at a tertiary care hospital in 2019. Case-patients were laboratory-confirmed measles with throat swabs tested by quantitative polymerase chain reactions (PCR), during April-May 2019. Medical histories and documented immunization records were obtained. We compared attack rates (ARs) among HCWs by occupational subgroup and age and examined the outbreak-associated costs. The index case was not ascertained. Among 26 measles case-patients (22 HCWs, four inpatients) aged 18-28 years, 25 had previously received measles-mumps-rubella (MMR) vaccine (12/26, 46% (two doses); 13/26, 50% (one dose)), and 16 (62%) had positive results of measles IgG prior to measles diagnosis. ARs were higher among HCWs aged 30 years (1.88%), especially in the subgroup under 25 years of age (2.22%). Control measures included work restrictions for seronegative HCWs (218/2320, 9.4%) in immunity verification, administration of the MMR vaccine (207 HCWs) or intravenous immunoglobulin (2 HCWs and 11 inpatients), enhanced health surveillance of HCWs, and mandatory assessment of patients with measles-like symptoms at the infectious diseases screening units. The hospital spent 90,417,132 Korean won (US $79,733) in response to the outbreak. Measles outbreaks can occur in healthcare settings despite high population immunity, highlighting the importance of stronger vaccination policies, particularly among young HCWs. Moreover, an effective outbreak response comprising immunization activities and enhanced surveillance of HCWs and patients to rapidly detect measles-like symptoms at a prodromal phase is essential to control nosocomial measles outbreaks.