Revaccination With Measles-Mumps-Rubella Vaccine and Infectious Disease Morbidity: A Danish Register-based Cohort Study.

Background: It has been hypothesized that revaccination with live vaccines is associated with reductions in off-target morbidity and mortality. We examined if revaccination with the live measles, mumps, and rubella vaccine (MMR) is associated with a lower rate of off-target infections. Methods: We performed a register-based nationwide cohort study that included 295559 children born in Denmark from April 2004 to December 2010. The cohort were followed from age 47 months (1 month before turning age 4 years, which is the recommended age of the second MMR [MMR-2]) until age 60 months. In Cox regression, we estimated adjusted incidence rate ratios (aIRRs) of antibiotic prescriptions and hospital admissions for any infection comparing MMR-2 as most recent vaccine with not having MMR-2 as the most recent vaccine. Results: There was no association between MMR-2 and antibiotic prescriptions (aIRR, 1.01; 95% confidence interval [CI], 0.99-1.02). The aIRR for the association between MMR-2 and admissions for infection of any duration was 0.93 (95% CI, 0.88-0.98). For admissions for infection lasting 0 to 1 day, the aIRR was 0.97 (95% CI, 0.90-1.03) compared with the aIRR of 0.84 (95% CI, 0.74-0.95) for admissions for infection lasting 2 days or longer (test for equality of aIRRs, P = .039). Conclusions: In this study, revaccination with MMR appeared safe in relation to off-target infections and was associated with a lower rate of severe off-target infections. More studies of the possible association between revaccination with live attenuated vaccines and off-target infections are needed.