aa Nichol 1995.

Methods	Randomised controlled trial conducted in the USA during the 1994 to 1995 influenza season. Follow‐up lasted from 1 December 1994 through to 31 March 1995. Influenza period was not defined. Virological surveillance was not performed.
Participants	841 full‐time employed: 419 treated and 422 placebo. Age of participants was 18 to 64.
Interventions	Subvirion, trivalent, parenteral influenza A and B vaccine. Schedule and dose were: single dose; 15 µg each strain. Vaccine composition was: A/Texas/36/91, A/Shangdong/9/93, B/Panama/45/90. Placebo was vaccine diluent. Vaccine was recommended and matched circulating strain.
Outcomes	Cases (symptom‐defined), working days lost due to respiratory illness, side effects. Participants were defined as cases if they had at least 1 upper respiratory illness (a sore throat associated with either fever or cough that lasted at least 24 hours). Local adverse effects were defined as arm soreness. Systemic adverse effects were defined as fever, tiredness, "feeling under the weather", muscle ache, headache (within a week after vaccination). Surveillance was active.
Notes	Circulating strain was not indicated. Efficacy and safety data were extracted. Industry funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was performed according to a computer‐generated randomisation schedule.
Allocation concealment (selection bias)	Low risk	Probably adequate
Blinding (performance bias and detection bias) All outcomes	Low risk	Double‐blinding was ensured by preloaded, coded, identical‐looking syringes.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Adequate
Summary assessment	Low risk	Adequate