The vaccine schedule has not been tested.
This is according to the Institute of Medicine (IOM) systematic review entitled “The Childhood Immunization Schedule and Safety: Stakeholder Concerns, Scientific Evidence, and Future Studies (2013)”, a report which is cited by the CDC.
Here is the full quote:
In summary, few studies have comprehensively assessed the association between the entire immunization schedule or variations in the overall schedule and categories of health outcomes, and no study has directly examined health outcomes and stakeholder concerns in precisely the way that the committee was charged to address in its statement of task. No studies have compared the differences in health outcomes that some stakeholders questioned between entirely unimmunized populations of children and fully immunized children. Experts who addressed the committee pointed not to a body of evidence that had been overlooked but rather to the fact that existing research has not been designed to test the entire immunization schedule.
Vaccines have been tested, but the vaccine schedule has not been.
A test of the vaccine schedule would entail comparing health outcomes between populations given different combinations of vaccines, including fully unvaccinated, selectively vaccinated, and fully vaccinated according to the recommended schedule (which is different in different countries). As the IOM quote above makes clear, there are no such studies.
Retrospective cohort and case-control studies comparing health outcomes between populations vaccinated using different schedules are urgently needed.