Author Contributions: Dr Sun had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Sun, Christensen, Hviid, Olsen, Vestergaard.
Acquisition of data: Li, Vedsted, Olsen, Vestergaard.
Analysis and interpretation of data: Sun, Christensen, Hviid, Li, Vedsted, Olsen, Vestergaard.
Drafting of the manuscript: Sun.
Critical revision of the manuscript for important intellectual content: Sun, Christensen, Hviid, Li, Vedsted, Olsen, Vestergaard.
Statistical analysis: Sun.
Obtained funding: Sun, Li, Olsen.
Administrative, technical, or material support: Li, Vestergaard.
Study supervision: Olsen, Vestergaard.
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.
Funding/Support: This study was supported by a grant 195/04 from the Lundbeck Foundation. Dr Sun also received support from the Lennart Grams Memorial Foundation. The data recruitment was supported by grants 271-07-0437 and 09-072986 from the Danish Medical Research Council, grant 070331 from NordForsk, and Seventh Framework Programme (FP7) grant 206242 from the European Research Council.
Role of the Sponsors: The funding sources had no role in the design and conduct of the study, in the collection, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript.
Previous Presentation: Presented in part as a poster at the 3rd North American Congress of Epidemiology; June 21-24, 2011; Montreal, Quebec, Canada.
Additional Contributions: Morten Frydenberg, PhD (Section for Biostatistics, Aarhus University, Aarhus, Denmark), provided discussions on confounding by delayed vaccination among sick children and helped on statistical issues. Dr Frydenberg was not paid for his contribution.