aa Bridges 2000b.

Methods	Randomised controlled trial, double‐blind, conducted in the USA during the 1998 to 1999 influenza season. Follow‐up lasted from November to March. The influenza period was defined as the period during which clinical specimens collected from ill participants yielded influenza viruses (4 January 1998 through 14 March 1999) and lasted 10 weeks. Pharyngeal swabs and paired sera were collected from ill people.
Participants	1191 healthy factory employees: 587 treated and 604 placebo. Age of participants was 19 to 64.
Interventions	Commercial trivalent, inactivated, intramuscular vaccine. Schedule and dose were not indicated. Vaccine composition was: A/Beijing/262/95, A/Sydney/5/97, and B/Harbin/7/94. Placebo was sterile saline for injection. Vaccine was recommended and matched 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/Sydney/5/97‐like and B/Beijing/184/93‐like. Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Volunteers were randomly allocated to receive vaccine or placebo using a table of random numbers.
Allocation concealment (selection bias)	Low risk	Adequate
Blinding (performance bias and detection bias) All outcomes	Low risk	Placebo was sterile saline for injection. Probably 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