| Study characteristics |
| Methods |
Prospective outbreak investigation study conducted in the USA during the 1967 to 1968 influenza season. Authors investigated an outbreak in a nursing home. Follow‐up period was December 1967 and January 1968. Throat swab and sera specimens were obtained from all ill residents and from an additional group of 27 residents with no illness. |
| Participants |
176 nursing home residents (131 treated and 31 controls were included in the analysis, data on immunisation status for 14 residents were not available) |
| Interventions |
Parenteral influenza vaccine: A2/Japan/170/62; A2/Taiwan/1/64; B/Massachusetts/3/66. Matching between vaccine and circulating strains is unknown. |
| Outcomes |
Clinically defined ILI (fever 37.8 °C or greater, headache, cough, sore throat, myalgia, and prostration) |
| Notes |
Poor reporting; no confounders control. The circulating strain was A2/Wis/1/68. |
| Risk of bias |
| Bias |
Authors' judgement |
Support for judgement |
| Allocation concealment (selection bias) |
High risk |
C ‐ Inadequate |
