Summary of findings 1. Influenza vaccine compared to placebo for preventing influenza in the elderly.
| Influenza vaccine compared to placebo for preventing influenza in the elderly | ||||||
| Patient or population: people aged over 65 years Setting: community and residential care institutions in the USA and Europe during influenza seasons between 1965 and 2000 Intervention: influenza vaccine Comparison: placebo | ||||||
| Outcomes | Anticipated absolute effects* (95% CI) | Relative effect (95% CI) | № of participants (studies) | Quality of the evidence (GRADE) | Comments | |
| Risk with placebo | Risk with influenza vaccine | |||||
|
Influenza assessed with: laboratory confirmation Follow‐up was conducted over an influenza season. |
Study population1 | RR 0.42 (0.27 to 0.66) | 2217 (3 RCTs) | ⊕⊕⊝⊝ LOW 2 3 | ||
| 57 per 1000 | 24 per 1000 (15 to 38) | |||||
|
Influenza‐like illness
assessed with: subjective report Follow‐up was conducted over an influenza season. |
Study population1 | RR 0.59 (0.47 to 0.73) | 6894 (4 RCTs) | ⊕⊕⊕⊝ MODERATE 2 | ||
| 59 per 1000 | 35 per 1000 (28 to 43) | |||||
|
Pneumonia Follow‐up was conducted over an influenza season. |
No events occurred in 1 study of 699 people. | ‐ | 699 (1 RCT) | ⊕⊝⊝⊝ VERY LOW 4 5 | ||
| Hospitalisations ‐ not reported | ‐ | ‐ | ‐ | ‐ | ‐ | |
|
All deaths Follow‐up was conducted over an influenza season. |
Study population1 | RR 1.02 (0.11 to 9.72) | 699 (1 RCT) | ⊕⊝⊝⊝ VERY LOW 4 5 | ||
| 6 per 1000 | 6 per 1000 (1 to 55) | |||||
|
Fever Follow‐up was conducted over an influenza season. |
Study population1 | RR 1.57 (0.92 to 2.71) | 2519 (3 RCTs) | ⊕⊕⊕⊝ MODERATE 6 | ||
| 16 per 1000 | 25 per 1000 (15 to 43) | |||||
|
Nausea Follow‐up was conducted over an influenza season. |
Study population1 | RR 1.75 (0.74 to 4.12) | 672 (1 RCT) | ⊕⊕⊝⊝ LOW 6 7 | ||
| 24 per 1000 | 42 per 1000 (18 to 98) | |||||
| *The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio | ||||||
| GRADE Working Group grades of evidence High quality: We are very confident that the true effect lies close to that of the estimate of the effect. Moderate quality: We are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low quality: Our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low quality: We have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect. | ||||||
1Control group risk taken as aggregate of the study control group risks. 2Downgraded one level due to serious risk of bias. Most of the evidence summarised in the meta‐analysis comes from studies with high or unclear risk of bias for more than one 'Risk of bias' domain. 3Downgraded due to indirectness. Uncertainty over the definition, testing, and surveillance of influenza in older trials. 4Downgraded two levels due to very serious imprecision. No events occurred in one study of nearly 700 people. The study was likely underpowered to detect effects on either pneumonia or mortality. 5Downgraded one level due to serious risk of bias. One study contributing data to this outcome had high risk of selection bias. 6Downgraded one level due to serious imprecision. Confidence intervals for nausea and fever were wide and include reduction and increase in risk of adverse events. 7Downgraded one level due to serious risk of bias. One study contributing data to this outcome had unclear risk of selection bias.