aa Leibovitz 1971.

Methods	Controlled clinical trial conducted in the USA during the 1969 to 1970 influenza season. The study period was 30 January to 18 May. Follow‐up lasted first 7 weeks of training. Influenza was detected from 11 February to 13 May and lasted 6 weeks. Participants were allocated to vaccine or control group according to the last non‐zero digit of the Social Security number. Blinding was not mentioned. Specimens for culture and acute‐convalescent blood specimens were obtained from people hospitalised with acute respiratory disease.
Participants	9616 military trainees: 1682 treated and 7934 placebo. Age of participants was 18 to 20.
Interventions	Monovalent inactivated, experimental, intramuscularly administered vaccine. Schedule and dose were: single dose, 556 CCA. Recombinant virus derived from HK/Aichi/68 and A0/PR8/34 was compared against no vaccination. Vaccine was not recommended but matched circulating strain.
Outcomes	Outcomes were: hospitalisation for upper respiratory infection (without definition), hospitalisation for influenza. Laboratory confirmation was based on culture and/or 4‐fold or greater rise in antibody titre occurring between acute and convalescent sera. Surveillance was passive.
Notes	Recruitment and immunisation period overlapped outbreak period. Most of the illnesses were due to adenovirus. Illnesses during the first 1 or 2 weeks after vaccination were not excluded, but the authors stated that this fact did not affect the results. Efficacy data only were extracted. Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Unclear
Allocation concealment (selection bias)	High risk	Inadequate
Blinding (performance bias and detection bias) All outcomes	Unclear risk	Unclear
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Unclear
Summary assessment	High risk	Unclear