BACKGROUND: In Denmark, live measles, mumps, and rubella vaccine (MMR) is associated with a reduced risk of infectious disease admissions, particularly for lower respiratory tract infections. In low-income countries, simultaneous vaccination (i.e. vaccination at the same visit) with live and inactivated vaccines may increase child mortality compared with the live vaccine alone. We examined the hypothesis that simultaneous administration of MMR and the inactivated DTaP-IPV-Hib vaccine compared with MMR alone is associated with higher incidence of infectious disease admissions.
METHODS: Nationwide, retrospective, register based cohort study of 520,859 children born in Denmark 1997-2006, who were followed from 15months to 4years of age. Incidence rate ratios (IRRs) of hospital admissions were estimated by Cox regression and adjusted for background factors including exact age.
RESULTS: By 2years of age, 4965 children had simultaneous MMR and DTaP-IPV-Hib as their most recent vaccination. Compared with MMR alone, simultaneous administration was associated with a higher rate of lower respiratory tract infections (adjusted incidence rate ratio (IRR), 1.27; 95% confidence interval (CI), 1.13-1.42). There was no effect on other infections. Overall, simultaneous administration was associated with a 7% (95% CI, 0-15%) increase in infectious disease admissions.
CONCLUSIONS: Simultaneous administration of MMR and DTaP-IPV-Hib compared with MMR alone may increase the rate of hospital admissions related to lower respiratory tract infections. These findings require replication in other high-income settings.