Multicomponent vaccines against as many as six infectious diseases are often given simultaneously with monovalent or multivalent vaccines against meningococcus and pneumococcus. Detailed analysis of simultaneous vaccinations, which can lead to suboptimal induced immune responses, should precede their combined use in national immunization programmes. By combining the results of the only two published studies evaluating simultaneous vaccinations with Infanrix hexa and Prevanar in a random-effect meta-analysis, we show that the one-sided 95% confidence interval of the HBV seroconversion rate includes the 95% limit that should be reached in universal programs. We advocate better assessment of potential interferences in immune responses after simultaneous vaccinations. Registration of new vaccines should include this assessment.