aa Mesa Duque 2001.

Methods	Randomised controlled trial, double‐blind, conducted in Colombia during the 1997 influenza season. Follow‐up lasted from 15 March to 31 August. Influenza period was not defined. Virological surveillance was not performed.
Participants	493 bank employees: 247 treated and 246 placebo. Age of participants was 18 to 60.
Interventions	Subunit inactivated, intramuscularly administered vaccine. Schedule and dose were: single dose. Vaccine composition was: A/Wahan/359/95, A/Texas/36/91, and B/Beijing/184/93. Placebo was vitamin C. Vaccine was recommended and matched circulating strain.
Outcomes	Episodes of clinical illness, WDL, and adverse effects. Clinical disease was defined as upper respiratory illness (fever, sore throat, and cough lasting more than 24 hours) according to ICD‐9 codes 381, 382, 460, 466, 480 and from 487 to 490. Local adverse effects were oedema, erythema, pain, and swelling. Systemic adverse effects were fever, headache, and indisposition within 5 days of vaccination. Surveillance was passive.
Notes	Circulating strains were not isolated from local cases but by WHO and Colombia surveillance system and matched vaccine components. Working days lost were detected all year round, so they were not included in the analysis. Efficacy and safety data were extracted. Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Volunteers were randomly allocated to receive vaccine or placebo using a table of random numbers.
Allocation concealment (selection bias)	Low risk	Given details provided regarding randomisation process and other aspects of the study design, we believe the allocation concealment was probably adequate.
Blinding (performance bias and detection bias) All outcomes	Low risk	Double‐blinding was ensured by pre‐labelled, coded, identical‐looking vials.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Adequate
Summary assessment	Low risk	Low risk