Table 1.

 Sources for cases, controls, exposure, and covariates by country in study of association between pandemic influenza A (H1N1) 2009 vaccine and Guillain-Barré syndrome

Country	Cases	Controls	Matching	Exposure to H1N1 vaccination	Covariates
UK	Each case identified from GPRD by using appropriate READ codes (F370.00, F370000, F370100, F370200, F370z00). Case verification done with free text, scanned hospital letters as well as GPs’ notes regarding diagnostic procedures. No major selection bias expected	Controls selected randomly from GPRD, matched on age, sex, practice, and index date to case. Consent not required. No selection bias expected	Sex, age, practice, index date	From automated GP records, no recall bias. Non-differential misclassification possible as some people might have been vaccinated outside of GP office	From GP record for cases and controls
Denmark	Cases identified from National Patient Register with primary discharge diagnoses only (ICD-10 code: G61.0). Case verification done after obtaining charts from cases. Potential small selection bias because of incomplete availability of charts	Controls selected randomly from Danish civil registration system. Up to 20 controls matched to case on age, sex, and index date. Consent not required. No selection bias expected	Sex, age, index date	From vaccination registry, no recall bias	From medical charts for cases only)
Netherlands	Cases identified prospectively through neurologists. Completeness verified retrospectively by checking against claims codes in each of reporting hospitals. Consent not required. Missing patients included retrospectively as far as possible. Verification of reporting against vaccination status showed incomplete reporting but non-differential regarding vaccinations	Controls were identified from GP of case patients. 10 patients randomly selected from list of registered patients and matched to case on age, sex, index date, and practice. Consent not required. No selection bias expected	Sex, age, practice, index date	From GP record. No recall bias Non-differential misclassification possible as some people might have been vaccinated outside of GP office	From GP records for cases and controls
France	Cases identified prospectively through neurologists in seven reference hospitals in France. Patients needed to provide informed consent. Completeness verified against pharmacy data (immunoglobulin prescriptions) and showed incomplete reporting (<50%). Vaccination status of non-reported cases could not be verified as linkage to vaccination registry required consent	Controls obtained from trauma unit in same hospital. Controls needed to provide informed consent. Response incomplete. Not possible to assess whether non-response differed by vaccination status and hence cannot exclude selection bias	Sex, age, hospital, index date	From registry. No recall bias	Medical records for both cases and controls
Sweden	Cases identified through seven neurology assessment laboratories where cases are laboratory confirmed for population of 9.4 million. Informed consent needed from all cases. Completeness of cases was checked in National Patient Registry for part of country. Recruitment incomplete because of delays in consent and non-consent. Not possible to assess whether this non-response differed by vaccination status and hence cannot exclude selection bias	Controls selected randomly from Swedish national population registry. Controls needed to provide informed consent. Response incomplete. Not possible to assess whether non-response differed by vaccination status and hence cannot exclude selection bias	Sex, age, index date	By interview, cannot exclude recall bias	By interview for cases and controls. Charts reviewed for case verification

GPRD=General Practice Research Database.