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Measles is known as a highly contagious vaccine-preventable disease. In Korea, a vaccination against the measles had been done since 1965. The measles, mumps, and rubella (MMR) vaccination to children was accelerated in South Korea as the National Immunization Programs (NIP) in 1985. Furthermore, a two-dose MMR vaccination schedule, which was introduced in 1997, contributed the decrement of measles among children and adults. Thanks to the national efforts to control measles, the World Health Organization declared that measles had been eliminated in South Korea in 2014 [1]. However, since 1996 outbreaks have occurred in nosocomial transmission after importation from overseas countries even in the situation in which 2-dose MMR vaccination coverage was over 95% [23]. Previous reports suggested that measles susceptibility is potentially increasing because of waning levels of measles IgG with increasing time post-vaccination in general population and young Korean healthcare workers (HCWs) [45]. However, there have been a few representative real-world data about the anti-measles IgG seropositive rate of Korean HCWs, even knowing anti-measles seropositive status of HCWs is thought as a minimum condition to keep each hospital operable in vaccine-preventable viral epidemics.
From December 2018 to January 2019, sixteen laboratories confirmed measles cases (8 children and 8 adult patients) occurred in Daegu, Korea, and 5 of 8 adult patients were HCWs of the same hospital. To prevent nosocomial transmission in the middle of measles outbreak, Daegu City financially supported the anti-measles IgG tests of 6 teaching hospitals' HCWs, and each hospital could complete vaccination of MMR for seronegative HCWs. From January 7, 2019 to January 23, 2019, 6 teaching hospitals in Daegu city had tested anti-measles IgG of patient-facing HCWs (such as physicians, residents, nurses in wards or outpatient clinics, etc.). Anti-measles IgG was detected using chemiluminescence immunoassay (LIAISON® Measles IgG, DiaSorin, Saluggia, Italy) on LIAISON® Analyzer system according to the manufacturer's instructions. Borderline or equivocal value of anti-measles IgG was treated as negative. About a quarter of HCWs (working in children's hospital and intensive care units) in hospital B was vaccinated twice with MMR during September 2018 to November 2018, these HCWs were excluded in this serological study. Chi-square or Fisher's exact tests were used to analyze the categorical variables. A P value less than 0.05 was considered statistically significant. R version 3.3.2 was used for the all statistical analysis. The study was approved by the Institutional Review Board of the Kyungpook National University Hospital (Technical opinion number: KNUH 2019-01-037).
The seroprevalence of anti-measles IgG was analyzed in a total of 9,132 HCWs among 6 teaching hospitals, and the seropositive rate was 75.9% (Table 1). The proportion of HCWs who were seronegative to measles was lowest in 20 - 29 year-old HCWs (n = 1,346, 40.1%) and highest in over fifty year-old HCWs (Table 1); this trend was observed in all hospitals (Fig. 1 and 2). In the hospital A and B, the proportions of seropositivity in twenties of HCWs (81.9% and 85.5%, respectively) were statistically higher than those of hospital C to F (47.0 - 57.1%) (P <0.0001). (Fig. 2). It could be because hospital A and B had made a temporary financial support for MMR to women HCWs who wanted MMR in 2001 to 2002 due to measles outbreak in 2000-2001 in Korea, and some colleges of nursing have a policy of MMR and varicella vaccinations before nursing practicum. It is possible that these vaccine policies of 2 hospitals made these differences in the proportion of anti-measles seropositivity especially in young age groups, compared to other hospitals. Hospital D had a temporary policy that the new HCWs should get MMR before starting their work in 2007 to 2015. However, other hospitals (C, E, and F) didn't have any specified policy about MMR.
According to Adult immunization schedule recommendation by Korean Society of Infectious Diseases in 2017 and 2012, all HCWs was recommended to get MMR vaccinations before starting their duties as a HCW [6]. However, this recommendation could be thought not to have been executed appropriately, due to financial problem (about 24,000 KRW \ [~22 USD] per person), lack of knowledge, and other causes, etc. In the clinical field, it is very important for the managers of hospital including infection control team to know the seropositivity rate of HCWs in each hospital, establish a budget for serologic tests and vaccinations, and deploy staff to more or less exposable parts to measles according to their serologic status. First, our report just shows preliminary results about the seropositivity of part of HCWs, not total employees of each hospital, because Daegu City supported only the cost of serologic tests of HCWs making direct contacts with patients (physicians and nurses). Second, the precise MMR vaccination histories of each HCW were not included in this study. However, the total number of enrolled HCWs in this study is larger than other previous domestic and foreign reports [478], and the very short period (<14 days) during which the serologic test results had been done could help to exclude other confounding factors like immune-potentiation by unexpected natural exposure to virus.
In summary, our result, as a previous suggestion by Kim et al. [4], supports that the routine MMR vaccination or serologic screening about measles should be required before entering the path of a HCW career in young Korean HCWs. To prevent nosocomial spread of measles to susceptible people, each hospital should get information about the status of protective antibody of their employees and make their own data periodically about anti-measles IgG seropositivity of their HCWs. Furthermore, these strategies should be expanded to other epidemic viral diseases, such as varicella, to which HCWs are at an increased risk of being exposed.
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