Challenges in comparing the safety of different vaccination schedules.

As vaccine hesitancy has increased in the United States, various authors have begun proposing alternatives to the Advisory Committee on Immunization Practices' recommended childhood immunization schedule. Because parents may believe the safety claims made by such authors, evaluations of the safety of alternative vaccination schedules are needed. However, comparing the safety of different vaccination schedules has numerous methodologic challenges. These challenges include defining vaccination history, defining safety, appropriately modeling interactions between vaccines, and appropriately handling age effects. Failure to properly address these challenges can result in biased results.