same detail as were original research papers. The searches initially yielded 748 references. This number was further reduced to 143 by exclusion of articles that reviewed vaccines not included in the current or recent childhood immunization schedule or included vaccines for adolescents, such as the human papillomavirus vaccine, and by elimination of references duplicated in more than one category. The number of articles reviewed was further reduced by limitation of the search to articles describing studies that examined at least one health outcome and at least one of the following elements of the schedule, including

  • number of vaccines,
  • frequency of administration,
  • spacing between doses,
  • cumulative doses,
  • age of the recipient, and
  • order of vaccine administration.

Though the committee did not undertake a formal systematic review, the quality of individual articles was judged by the validity of the study design, the method by which the research was conducted, and the transparency of methods. In the end, 37 articles were chosen, and these, organized by category of health outcome, are briefly summarized below.

A second search was performed by use of the MeSH “immunization schedule” without predefined headings to investigate specific diseases or conditions. This search was conducted to ensure that the committee’s review adequately addressed any demonstrated associations between components of the immunization schedule and adverse health outcomes. Again, the search was limited to articles published in the past 10 years and excluded reviews, commentaries, editorials, and similar publications. After application of the exclusionary criteria, 1,235 abstracts were reviewed, and this number was narrowed to 56 that were considered potentially relevant to the committee’s charge. The committee concluded that only four of these research papers covered aspects of the childhood immunization schedule and safety. Two were considered not helpful to an evaluation of safety. (One was an economic evaluation of the childhood immunization schedule and did not examine safety; the second had serious limitations and was not considered for this chapter.) Two of the papers provided useful information, so summaries are included under the appropriate outcome section below (one is included under allergy/atopy; the second is included under neurological outcomes).

A third search was done to examine studies of immunization in infants born prematurely. Although prematurity is not a “health outcome,” the committee’s efforts included collection of data on premature infants because

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