MMR and the EMR: Hunting for National-level Vaccine Data in Transplant Recipients

The growing threat posed by vaccine-preventable infections such as measles, mumps, and rubella has been highlighted by the resurgence of measles in the United States in 2025, in conjunction with declining vaccination rates.1,2 Although these outbreaks portend broad societal risk, the alarming consequence for immunocompromised people is the weakening of the circle of protection around them. Vaccines remain key, but there are safety constraints; except for carefully selected pediatric solid organ transplant recipients (SOTRs),3 live measles-mumps-rubella (MMR) vaccine historically cannot be given posttransplant or within 30 d before transplant.4 Transplant recipients are at risk for severe disease if they become infected, and the duration of posttransplant immunity is uncertain. An understanding of pretransplant seroprevalence for measles, mumps, and rubella, and incidence and outcomes of these infections posttransplant, would be important for designing strategies to mitigate risk. We lack national-level data in these domains, which interferes with our ability to keep SOTRs safe from these vaccine-preventable illnesses. In this issue of Transplantation, Chiu et al present an analysis of the retrospective electronic medical record (EMR)-based TriNetX database, to evaluate on a national level the IgG seroprevalence to measles, mumps, and rubella in adult solid organ transplant (SOT) candidates who were tested pretransplant.5 The authors also sought to describe the incidence and outcomes of infections with these 3 pathogens post SOT, particularly in a subgroup who received MMR vaccination within 30 d before transplant (typically in the setting of negative IgG or undervaccinated status). The authors identified 313,548 SOTRs between 2009 and 2024, spanning many regions of the United States and demonstrating a key strength of this national federated database. In keeping with the general SOT population, 62% were kidney recipients, 39% were born before 1957, and median age was 60 y (interquartile range 41–63 y). Only about 8%, however, had record of an IgG serologic test result for ≥1 of measles, mumps, or rubella pre-SOT (~4%–5% for each pathogen), using a variety of identified tests. Thoracic recipients, particularly lung recipients, were more likely to have had 1 of these tests performed than other SOTRs, as were people born after 1957. For those who were tested, there were reasonable overall seroprevalence rates of 72%–80%, albeit lower versus measles, especially in younger persons (69% versus 84%). Chiu et al identified a total of 269 measles, mumps, and rubella infections using ICD codes (