Is vaccination against measles, mumps, and rubella associated with reduced rates of antibiotic treatments among children below the age of 2 years? Nationwide register-based study from Denmark, Finland, Norway, and Sweden

•MMR vaccinated children had lower rates of antibiotic treatments than children with no MMR.•Bias may explain at least some of the observed association.•Limited non-specific effects of MMR vaccine on infections treated out-of-hospital. Previous studies have shown that vaccination against measles, mumps, and rubella (MMR) may have beneficial non-specific effects, reducing the risk of infections not targeted by the vaccine. We investigated if MMR vaccine given after the third dose of diphtheria-tetanus-acellular pertussis vaccine (DTaP3), was associated with reduced rates of antibiotic treatments. Register-based cohort study following children from the age of recommended MMR vaccination until age 2 years. We included 831,287 children born in Denmark, Finland, Norway, and Sweden who had received DTaP3 but not yet MMR vaccine. Cox proportional hazards regression with age as the underlying timescale and vaccination status as a time-varying exposure was used to estimate covariate-adjusted Hazard Ratios (aHRs) and inverse probability of treatment weighted (IPTW) HRs of antibiotic treatments. Summary estimates were calculated using random-effects meta-analysis. Compared with only having received DTaP3, receipt of MMR vaccine after DTaP3 was associated with reduced rates of antibiotic treatments in all countries: the aHR was 0.92 (0.91–0.93) in Denmark, 0.92 (0.90–0.94) in Finland, 0.84 (0.82–0.85) in Norway, and 0.87 (0.85–0.90) in Sweden, yielding a summary estimate of 0.89 (0.85–0.93). A stronger beneficial association was seen in a negative control exposure analysis comparing children vaccinated with DTaP3 vs two doses of DTaP. Across the Nordic countries, receipt of MMR vaccine after DTaP3 was associated with an 11% lower rate of antibiotic treatments. The negative control analysis suggests that the findings are affected by residual confounding. Findings suggest that potential non-specific effects of MMR vaccine are of limited clinical and public health importance for the milder infections treated out-of-hospital in the Nordic setting.