aa Bridges 2000a.

Methods	Randomised controlled trial, double‐blind, conducted in the USA during the 1997 to 1998 influenza season. Follow‐up lasted from November to March. Influenza period was defined as the period during which clinical specimens collected from ill participants yielded influenza viruses (8 December 1997 through 2 March 1998) and lasted 12 weeks. Volunteers were randomly allocated to receive vaccine or placebo using a table of random numbers. Pharyngeal swab and paired sera were collected from ill people.
Participants	1184 healthy factory employees: 595 treated and 589 placebo. Age of participants was 18 to 64.
Interventions	Commercial trivalent, inactivated, intramuscular vaccine. Schedule and dose were not indicated. Vaccine composition was: A/Johannesburg/82/96, A/Nanchang/933/95, and B/Harbin/7/94. Placebo was sterile saline for injection. Vaccine was recommended but did not match the circulating strain.
Outcomes	Influenza‐like illness, influenza, days ill, physician visits, times any drug was prescribed, times antibiotic was prescribed, working days lost, admissions, adverse effects. They were defined as follows: influenza‐like illness: fever = 37.7 °C with cough or sore throat); upper respiratory illness: cough with sore throat or fever = 37.7 °C. Local adverse effects were arm soreness and redness. Systemic adverse effects were: fever, sore throat, coryza, myalgia, headache, and fatigue, but authors reported no data. Surveillance was passive.
Notes	For analysis we chose the influenza‐like illness definition. Intention‐to‐treat analysis was performed. Systemic adverse effects were not reported. Circulating strain was A/Sidney/5/97‐like. Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Insufficient description
Allocation concealment (selection bias)	Low risk	Adequate
Blinding (performance bias and detection bias) All outcomes	Low risk	Adequate
Incomplete outcome data (attrition bias) All outcomes	Low risk	Attrition reasons for the whole cohort are provided by the participant flow.
Summary assessment	Low risk	Low risk of bias