bb Siscovick 2000.

Methods	Study assessing the association between influenza vaccination the previous year and the risk of primary (i.e. occurring in people with no previous history of cardiac disease) cardiac arrest. Case‐control study on 360 cases and 418 controls
Participants	Cases: people who had experienced primary cardiac arrest, aged between 25 and 74 years. Controls: healthy people selected randomly from the community, who were matched to the cases for age and sex.
Interventions	Immunisation with influenza vaccine, assessed by means of questionnaires
Outcomes	Cardiac arrest
Notes	The authors concluded that vaccination is protective against primary cardiac arrest (OR 0.51, 95% CI 0.33 to 0.79). The difficulty of case ascertainment (77% of potential cases had no medical report and/or autopsy) and recall bias (spouses provided exposure data for 304 cases, while 56 survivor cases provided data jointly with their spouses) make the conclusions of this study unreliable. The reliability of this study is unclear due to a lack of detail on the circulation of influenza in the study areas in the 12 months preceding cardiac arrest (the causal hypothesis is based on the effects of influenza infection on the oxygen supply to the myocardium through lung infection and inflammation). Rare events (safety) Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
CC ‐ case selection All outcomes	Low risk	Cases of out‐of‐hospital PCA attended by paramedics in King County, Washington, from October 1988 to July 1994 were identified from paramedic incident reports. Primary cardiac arrest cases were defined by the occurrence of a sudden pulseless condition and the absence of evidence of a non‐cardiac condition as the cause of cardiac arrest.
CC ‐ control selection All outcomes	High risk	Selected from the community using random digit dialling
CC ‐ comparability All outcomes	Unclear risk	For each PCA case, 1 to 2 controls, matched for age (within 7 years) and sex
CC ‐ exposure All outcomes	Unclear risk	"Data on the participants’ vaccination status were collected from both case and control spouses by using a standardised questionnaire. For each participant, information was collected on whether they had received an influenza vaccination during the previous 12 months and, if so, when the vaccination had been given. We did not collect information on whether they had received influenza vaccination during the years prior to that period."
Summary assessment	High risk