aa Mogabgab 1970b.

Methods	Randomised study conducted in the USA during the 1968 to 1969 influenza season. Influenza outbreak lasted 9 weeks, from 9 December to 3 February. Randomisation methods were not described. Laboratory confirmation was obtained (by culture or 4‐fold antibody titre increase in acute convalescent sera) for 20 men randomly selected each week from among the ill.
Participants	1551 airmen previously unvaccinated: 1030 vaccinated and 521 given placebo. Age of participants was 18 to 21.
Interventions	Polyvalent inactivated influenza A and B vaccine (the 1967 military formula). Schedule and dose were: single dose. Vaccine composition was: A/Swine/33 100 CCA units, A/PR8/34 100 CCA units, A1/AA/1/57 100 CCA units, A2/Taiwan 1/64 400 CCA units, B/Lee/40 100 CCA units, B/Mass 3/66 200 CCA units. Placebo was saline for injection. Vaccine was recommended but did not match the circulating strain.
Outcomes	ILI and influenza cases, complications and admissions. All respiratory illnesses were classified as febrile (38.3 °C or greater), afebrile, pharyngitis, bronchitis, or pneumonia (complications). Surveillance was passive.
Notes	Cases occurring during the first 15 days after vaccination were not included in the analysis. Circulating strain was A2/Hong Kong. Efficacy data 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)	Unclear risk	Unclear
Blinding (performance bias and detection bias) All outcomes	Unclear risk	Unclear
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Unclear
Summary assessment	Unclear risk	Unclear