aa Nichol 1999a.

Methods	Randomised controlled trial conducted in the USA during the 1997 to 1998 influenza season. Follow‐up lasted from November to March. Site‐specific peak outbreak period was defined as weeks including 80% of the isolates of a specific area. Total outbreak period lasted from 14 December 1997 through to 21 March 1998. Total outbreak period was included in the analysis and lasted 14 weeks. Participants were recruited from 7 organisations and assigned to 1 of the study groups using a permuted block randomisation scheme that was stratified by treatment centre and age group. Sealed randomisation envelopes contained vaccine codes. Influenza virus surveillance was carried out in the area.
Participants	4561 healthy working adults: 3041 treated and 1520 placebo. Age of participants was 18 to 64.
Interventions	Trivalent, live attenuated influenza A and B vaccine in a single dose. Vaccine composition was: A/Shenzhen/227/95, A/Wuhan/395/95, B/Harbin/7/94‐like. Placebo was egg allantoic fluid. Vaccine was recommended but did not match the circulating strain.
Outcomes	Clinical cases (symptom‐defined), working days lost, and adverse effects. Case definition had 3 specifications: febrile illness (fever for at least 1 day and 2 or more symptoms for at least 2 days: fever, chills, headache, cough, runny nose, sore throat, muscle aches, tiredness); severe febrile illness (3 days of symptoms and 1 day of fever); febrile upper respiratory tract illness (3 days of upper respiratory tract symptoms and 1 day of fever). We chose the febrile illness outcome for analysis. Systemic adverse effects were defined as headache, muscle aches, chills, tiredness, and fever. Surveillance was passive.
Notes	Complete follow‐up data were obtained for 2874 participants in the treatment arm and 1433 participants in the placebo arm. The outcome working days lost is presented as a rate ratio; the data are presented in a way that allows us to compute the difference in mean days lost but not to compute the standard error. Circulating strain was A/Sydney/5/97‐like. Efficacy and safety data were extracted. Government and industry funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Adequate
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	Adequate
Summary assessment	Low risk	Adequate