What is the study design of the following study?

JAMA. Links to an external site.2015. Autism occurrence by MMR vaccine status among US children with older siblings with and without autism.

Abstract

IMPORTANCE: Despite research showing no link between the measles-mumps-rubella (MMR) vaccine and autism spectrum disorders (ASD), beliefs that the vaccine causes autism persist, leading to lower vaccination levels. Parents who already have a child with ASD may be especially wary of vaccinations.

OBJECTIVE: To report ASD occurrence by MMR vaccine status in a large sample of US children who have older siblings with and without ASD.

DESIGN, SETTING, AND PARTICIPANTS: A study using an administrative claims database. Participants included children continuously enrolled in the health plan from birth to at least 5years of age during 2001-2012 who also had an older sibling continuously enrolled for at least 6 months between 1997 and 2012.

EXPOSURES: MMR vaccine receipt (0, 1, 2 doses) between birth and 5 years of age.

MAIN OUTCOMES AND MEASURES: ASD status defined as 2 claims with a diagnosis code in any position for autistic disorder or other specified pervasive developmental disorder (PDD) including Asperger syndrome, or unspecified PDD (International Classification of Diseases, Ninth Revision, Clinical Modification 299.0x, 299.8x, 299.9x).

RESULTS: Of 95,727 children with older siblings, 994 (1.04%) were diagnosed with ASD and 1929 (2.01%) had an older sibling with ASD. For children with older siblings with ASD, at age 2, the adjusted relative risk (RR) of ASD for 1 dose of MMR vaccine vs no vaccine was 0.76 (95% CI, 0.49-1.18; P=.22), and at age 5, the RR of ASD for 2 doses compared with no vaccine was 0.56 (95% CI, 0.31-1.01; P=.052). For children whose older siblings did not have ASD, at age 2, the adjusted RR of ASD for 1 dose was 0.91 (95% CI, 0.67-1.20; P=.50) and at age 5, the RR of ASD for 2 doses was 1.12 (95% CI, 0.78-1.59; P=.55).

CONCLUSIONS AND RELEVANCE: In this large sample of privately insured children with older siblings, receipt of the MMR vaccine was not associated with increased risk of ASD, regardless of whether older siblings had ASD. These findings indicate no harmful association between MMR vaccine receipt and ASD even among children already at higher risk for ASD.