Ab titers to measles decrease over time in HSCT recipients. Among patients who were seropositive at the time of allogeneic HSCT, only 51% retained antibodies to measles 2 years later.13 In contrast, 1 year after autologous HSCT (autologous stem cell transplant recipient [ASCT]), 88% maintained seropositivity to measles.14 However, there are limited data on the seroprevalence of measles after ASCT in a high seroprevalence country such as South Korea. Therefore, we evaluated measles IgG approximately 1–2 years after ASCT and compared them to those of the control healthcare workers (HCWs).

This study was conducted at Asan Medical Center (AMC), a 2,700-bed tertiary hospital in Seoul, Korea. All adult patients aged 18 years and older with hematologic malignancy who received ASCT at AMC between January 2013 and December 2021 were retrospectively reviewed, and ASCT recipients whose measles Ab results after ASCT were available were enrolled. Data on measles IgG approximately 1–2 years after transplant were collected. Additionally, records of measles seropositivity among HCWs at the hospital from 2014 to 2024 were obtained. During the domestic epidemic of measles in 2014, our hospital conducted a measles Ab test on all existing employees to prevent the measles epidemic by forming herd immunity within the hospital. If measles Ab was negative, one-time measles, mumps, and rubella (MMR) vaccination was administered. Since 2014, new employees have been tested for measles antibodies as on-boarding protocol and have been vaccinated with MMR if measles Ab was negative. In 2018, a complete measles Ab survey was conducted again on certain employees whose measles immunity was not confirmed. Vaccination was administered for sero-negative HCWs, based on the Korea Disease Control and Prevention Agency guidelines. HCWs were matched to ASCT recipients at a 1:3 ratio based on age and sex as a healthy control. However, due to insufficient numbers of HCWs aged 60 and above, they were not matched and the seroprevalence was compared based on the actual numbers. The difference in IgG positivity rates was assessed using either the χ² test or Fisher’s exact test, as appropriate. P values of less than 0.05 were considered statically significant.

Among a total of 677 ASCT recipients, measles Ab results were available for 261 patients (38.6%) (Supplementary Fig. 1). Baseline clinical characteristics are presented in Supplementary Table 1. Of the 261 patients, 144 (55.2%) were male, with a median age of 56 years (interquartile range [IQR], 48–60). The most prevalent diseases for ASCT were non-Hodgkin’s lymphoma (NHL) (74.7%), multiple myeloma (MM) (16.8%), and Hodgkin's lymphoma (HL) (5.7%).

The positive rates of measles IgG are shown in Supplementary Table 2. Measles IgG were assessed approximately 1–2 years after ASCT (median, 11 months; IQR, 9–14). Among the 261 patients, 217 (83.1%) tested positive for measles IgG. In contrast, measles Ab was positive in 539 (90.3%) of the age- and sex-matched 597 HCWs. The seropositivity of measles Ab after ASCT was significantly lower than that in the control HCWs (P = 0.002). We compared measles IgG positive rate by age groups (Table 1). The difference of seropositive rate between post-ASCT and HCWs were statistically significant in age of 50 years and older; 87.5% (84/96) vs. 96.9% (279/288) in the age of 50s (P < 0.001) and 85.4% (76/89) vs. 98.8% (80/81) in the age of 60s (P = 0.001). In addition, the positive rate of measles Ab in ASCT recipients with lymphoma (HL or NHL) was higher (85.7% [180/210]) than that in those with MM (68.2% [30/44]) (P = 0.005) (Supplementary Table 2).

After the end of the coronavirus disease 2019 epidemic, measles cases are increasing around the globe, especially European Regions, according to the joint press release from World Health Organization and UNICEF.15 In 2023, there were over 300,000 cases of measles worldwide. The number of cases in 2024 is expected to match or exceed the total in 2023. Recently quarantine measures are being eased and overseas travel is increasing in Korea. There is a high possibility that measles cases will be imported from overseas. Our study revealed that over 80% of ASCT recipients retained antibodies to measles approximately 1–2 years post-transplant, aligning with findings from previous studies involving ASCT recipients.141617 However, the prevalence of IgG antibodies to measles after ASCT was significantly lower than that observed in the control HCWs. Considering the high contagiousness of measles and the potential for small outbreaks even in high-immunity populations, it is imperative to test Korean ASCT recipients for measles IgG after ASCT and administer vaccination if seronegative 2 years post-ASCT.

Measles IgG levels appeared to be more sustained in ASCT patients (70–88%) compared with allogeneic HSCT recipients (51–61%).131416171819 A study involving 57 patients revealed that at 2 years post-allogeneic HSCT, only 51% retained measles antibodies.13 Data from the Netherlands indicated a significant decline in measles immunity from 91% before allo-HSCT (85/91 patients) to 86% (67/78 patients) at 3 months after allo-HSCT, and further to 61% (55/84 patients) at 1 year post-transplant.14 In another study focusing on patients seropositive before allo-HCT, the probabilities of remaining seropositive at 1- and 2-years post-transplant were 75.0% (66.7–81.5%) and 60.6% (51.7–68.4%) for measles, respectively.18 In a cohort of 126 allogeneic HCT recipients transplanted between 1 and 39 years ago, 62% were seropositive/seroprotected for measles.19 Conversely, a prior study reported that 1 year after ASCT, 88% of patients maintained seropositivity to measles.14 Another study revealed that 1 year post-ASCT, 7 (12%) of 57 patients who were initially seropositive to measles became seronegative.16 Among those retested at 2 and 3 years, an additional three patients became seronegative for measles.16 Arribas et al.17 found that 77 (70%) of 110 patients with MM had positive measles titers approximately 100 days after ASCT. Therefore, our study is consistent with findings from previous studies involving ASCT recipients.141617

This study has several limitations. First, the number of patients who underwent measles IgG testing before ASCT was small because measles IgG testing before ASCT was not routinely performed during the study period, precluding the analysis of pre- and post-ASCT data in the same patients. Consequently, there were limited serial test results for measles IgG, preventing the evaluation of the kinetics of waning measles Ab after ASCT. Thus, the cautious interpretations are needed on our cross-sectional seroprevalence data due to the lack of the kinetics data. Second, we did not assess the patients’ vaccination records from childhood or the previous history of measles. Third, some may argue that HCWs may not be appropriate controls for ASCT recipients, as they are recommended to receive MMR vaccination if they do not have adequate presumptive evidence of immunity. However, the seropositive data from HCWs had been obtained before this recommendation was adopted in our hospital, so we assume that the seropositivity from HCWs might not be substantially higher than that from the general adult population. Lastly, we had to enroll HCWs in the age of 60 years and older into the control without 1:3 random matching due to limited number.

In conclusion, most ASCT recipients in our study exhibited seropositivity against measles, albeit at a lower rate compared with healthy controls. Our findings underscore the vulnerability of adult ASCT recipients to measles compared with the general Korean adult population. Clinicians should therefore prioritize testing for measles IgG after ASCT and consider vaccination for ASCT recipients who remain seronegative 2 years after ASCT.