bb‐Smeeth 2004.

Study characteristics
Methods	Case‐control study using the UK General Practice Research Database (GPRD)
Participants	The study population consisted of all people who were registered in the GPRD at any time between 1 June 1987 (when the database was started) and 31 December 2001, and who were born in 1973 or later, to ensure that virtually all individuals eligible for MMR vaccination were included. Cases: defined as children with a first diagnosis of a PDD during the study period whilst registered with a practice contributing to the GPRD. They were found by searching the electronic records for clinical codes indicating a diagnosis of PDD (codes used are available on request). Those who were first diagnosed outside the study period were excluded from the study and were not eligible to be selected as controls. Those with autistic disorders and similar presentations were classified as having “autism” and those with other descriptions (such as Asperger’s syndrome) were classified as having “other PDD”. Patients who had more than 1 PDD diagnostic code recorded at different times (e.g. autism and then Asperger’s syndrome) were classified as having the most specific diagnosis (in this example Asperger’s syndrome). However, the date of the first diagnosis with a PDD was taken as the date of diagnosis. Controls: 5 controls for every case from amongst individuals in the study population who had no diagnosis of PDD recorded in their general practice record and who were alive and registered with a participating practice on the date of the PDD diagnosis in the case. Controls were individually matched to cases by year of birth (up to 1 year older or younger), sex, and general practice.
Interventions	Exposure to MMR vaccination from birth to index date (date of the first diagnosis with PDD). In 1988, MMR vaccination was introduced in the UK for all children aged 12 to 15 months. During 1988 to 1991, in a catch‐up campaign, MMR vaccine was also offered to all children up until the age of school entry (4 to 5 years). A second dose at school entry was introduced in 1996, with a further catch‐up campaign for children born on or after 1 January 1990, who had not previously received 2 doses of a vaccine containing measles. MMR vaccination is also recommended for non‐immune adults, especially those in residential care or those starting college, and for non‐immune contacts during a measles outbreak. A catch‐up campaign for children aged 5 to 16 years was launched in 1994, but measles‐rubella vaccination was used, not MMR.
Outcomes	Number of MMR vaccination amongst cases and controls prior to PDD diagnosis and prior to PDD diagnosis and 3rd birthday
Funding Source	Government
Notes	The study method is described in Smeeth 2001.
Risk of bias
Bias	Authors' judgement	Support for judgement
CCS ‐ case selection	Low risk	Adequate ‐ secure record ‐ General Practice Research Database
CCS ‐ control selection	Low risk	Adequate ‐ community
CCS ‐ comparability	Low risk	Adequate ‐ matched for age, sex, general practices
CCS ‐ exposures	Low risk	Adequate ‐ secure record
Summary Risk of Bias assessment	Low risk	Plausible bias is unlikely to have seriously altered the results.