db‐O'Leary 2012.

Study characteristics
Methods	Self‐controlled case series methods to examine the risk of ITP after childhood vaccines
Participants	Children < 18 years This investigation was conducted in 5 healthcare systems (Kaiser Permanente: Colorado, Hawaii, Georgia, and Northern California, and Harvard Vanguard Medical Associates) using data from the years 2000 to 2009. Included children who had been vaccinated whilst actively enrolled in their respective health plans.
Interventions	MMR vaccine, MMRV vaccine DTaP (diphtheria‐tetanus‐acellular pertussis vaccine); HBV (hepatitis B virus vaccine); Hep A (hepatitis A vaccine); Hib (Haemophilus influenzae type b vaccine); HPV (human papillomavirus vaccine); IPV (inactivated poliovirus vaccine); MCV (meningococcal conjugate vaccine); PCV (pneumococcal conjugate vaccine); RV (rotavirus vaccine); Tdap (tetanus‐diphtheria‐acellular pertussis vaccine); TIV (trivalent influenza vaccine); VAR (varicella vaccine)
Outcomes	Identification of possible cases was conducted at the lead site by using electronic databases, with the analyst blinded to vaccination status. The authors reviewed the electronic data to exclude cases of thrombocytopenia from other known conditions by using the ICD‐9 diagnosis codes (such as neonatal thrombocytopenia, aplastic anaemia, disseminated intravascular coagulation, acquired haemolytic anaemia, chronic liver disease, or malignancy). Children < 18 years with either 2 platelet counts of 50,000/mL in a 6‐week period or 1 platelet count of 50,000/mL and an associated ICD‐9 code of 287.0 to 287.9, inclusive, within 6 weeks of the low platelet count were included. A case was excluded if, in the 6 weeks before diagnosis, the child was exposed to a platelet‐depleting medication (such as antiepileptics and sulfonamide antibiotics) or infected with wild‐type varicella or Epstein‐Barr virus.
Funding Source	Government
Notes	Follow‐up time: 365 days before and after vaccination Exposed period: 1 to 42 days after vaccination for all vaccines Unexposed period was defined as the time before and after the exposed period within 365 days of follow‐up before or after vaccination. Day 0 (the day of vaccination) was excluded, because any cases occurring at this time were most likely coincidental.
Risk of bias
Bias	Authors' judgement	Support for judgement
SCCS/PTC ‐ case selection	Low risk	Adequate ‐ computerised hospital record
SCCS/PTC ‐ exposure	Low risk	Adequate ‐ computerised child health and general practice records
SCCS/PTC ‐ observation and exposure risk period	Low risk	Adequate ‐ observation periods are well‐defined, exposure period appears to be well‐documented
SCCS/PTC ‐ comparability	Low risk	Adequate ‐ stratified for age
Summary Risk of Bias assessment	Low risk	Plausible bias is unlikely to have seriously altered the results.