bb Galeotti 2013.

Methods	Case‐control study testing the association between influenza vaccination and Guillain‐Barré syndrome
Participants	Cases (n = 140): adults with Guillain‐Barré syndrome defined according to the Brighton Collaboration definition (levels 1 to 3) recruited at 121 neurological centres in 7 Italian regions and having symptoms onset between 1 October 2010 and 15 May 2011. Controls (n = 308): were selected from among patients admitted to the emergency department of the same hospital as the cases for acute conditions unrelated to chronic diseases (e.g. trauma). Each control was individually matched to a case for admission date (i.e. the same date as the case or up to 30 days afterwards), sex, age (± 5 years), and region of residence.
Interventions	Exposure to influenza vaccination (date and brand of vaccine) was verified by contacting patients’ general practitioners by telephone. A neurologist (FG) closely verified and queried data quality.
Outcomes	Guillain‐Barré syndrome
Notes	The authors also performed data analysis with a controlled case series design, considering the 6 weeks following exposure as the risk time. Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
CC ‐ case selection All outcomes	Low risk	Consecutive series of cases
CC ‐ control selection All outcomes	Low risk	Hospital control
CC ‐ comparability All outcomes	Unclear risk	Matched analysis only for sex, age, region, admission date
CC ‐ exposure All outcomes	Unclear risk	Unclear if interviewers were blinded to case‐control status
Summary assessment	Unclear risk	Unclear risk of bias