| Study characteristics |
| Methods |
Prospective cohort study conducted in Michigan, USA during the 1991 to 1992 influenza season. Authors investigated 26 skilled nursing homes with evidence of flu activity; nursing homes with high rates of immunisation (herd immunity) were excluded from the study; data on ILI or pneumonia were recorded prospectively under supervision of a nurse co‐ordinator. Follow‐up period was 1 November 1991 to 29 February 1992. |
| Participants |
2351 residents in 26 nursing homes (1728 treated and 623 controls, all included in the analysis), 65 years or older, for whom vaccination status was known |
| Interventions |
Parenteral influenza vaccine. Vaccine strains matched circulating strains. |
| Outcomes |
Clinically defined ILI (fever 37.8 °C or greater + cough, sore throat, or nasal congestion), clinical pneumonia, deaths occurring within 3 months of the onset of respiratory illness. Influenza was considered to have been introduced into a nursing home when a least 2% of residents developed ILI within a 7‐day period during community‐documented virus circulation or when virus was isolated from cases. |
| Notes |
Both influenza A (H3N2) and A (H1N1) co‐circulated with influenza A (H3N2) predominantly. The circulating strains were closely related to the vaccine strain. Rate ratio estimates were adjusted by sex, age, home size and presented by "peak period". Groups were comparable as age and chronic conditions. |
| Risk of bias |
| Bias |
Authors' judgement |
Support for judgement |
| Allocation concealment (selection bias) |
Low risk |
A ‐ Adequate |
