| Study characteristics |
| Methods |
Experimental study conducted in California, USA during the 1965 to 1966 influenza season; the control group received influenza B vaccine, placebo, or no vaccine; laboratory samples were obtained from ill people to confirm the infection active surveillance. Follow‐up period was 1 February 1966 to 30 April 1966. |
| Participants |
4180 residents in the nursing home, healthy (1561 treated and 2619 controls were included in the analysis), 52 years or older |
| Interventions |
Monovalent A2 parenteral influenza vaccine: A2/Taiwan/1/64. Vaccine strains matched the circulating strains |
| Outcomes |
Clinically defined febrile illness (fever + cough or malaise or coryza or myalgia or headache), clinically defined afebrile illness, hospitalisation and deaths without definition.
Harms were reported, but they were excluded from analysis, as they refer to an old oil adjuvant vaccine |
| Notes |
Participants randomised the previous year but not vaccinated (reason not explained) in the current year were added in the control group; the study year was an epidemic one |
| Risk of bias |
| Bias |
Authors' judgement |
Support for judgement |
| Random sequence generation (selection bias) |
Unclear risk |
Insufficient details available to permit judgement |
| Allocation concealment (selection bias) |
Unclear risk |
Insufficient details available to permit judgement |
| Blinding (performance bias and detection bias)
All outcomes |
High risk |
Open‐label design |
| Incomplete outcome data (attrition bias)
All outcomes |
Low risk |
Data available for most participants in the study |
