Background: It has been hypothesized that revaccination with live vaccines could be 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: A register-based nationwide cohort study including 295,559 children born in Denmark 1 April 2004 to 31 December 2010. The cohort were followed from age 47 months (1 month before turning age 4 years, 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 among children who had received no non-live vaccines with or after MMR.

Results: 76.7% received MMR-2 before age 60 months. There was no association between MMR-2 and antibiotic prescriptions (aIRR, 1.01; 95% confidence interval (CI), 0.99 to1.02). The aIRR for the association between MMR-2 and admissions for any infection of any duration was 0.93 (95% CI, 0.88 to 0.98). For admissions for infection lasting 0 to 1 day, the aIRR was 0.97 (95% CI, 0.90 to1.03) compared with the aIRR of 0.84 (95% CI, 0.74 to 0.95) for admissions for infection lasting 2 days or longer (Test for equality of aIRRs, p=0.039).

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 are needed of the possible association between revaccination with live attenuated vaccines and off-target infections.