Eighteen cohort studies examined the effect of influenza vaccination on respiratory illness (4,368 vaccinated and 4,944 unvaccinated patients).
Nine cohort studies examined the effect of influenza vaccination on the occurrence of pneumonia (5,938 vaccinated and 18,836 unvaccinated patients).
Six cohort studies examined the effect of influenza vaccination on hospitalisation (5,476 vaccinated and 18,848 unvaccinated patients).
Twenty cohort studies examined the effect of influenza vaccination on mortality (8,302 vaccinated and 21,726 unvaccinated patients.
Eighteen cohort studies examined vaccine efficacy when the epidemic strain was the same or a drift variant (4,357 vaccinated and 5,119 unvaccinated).
The pooled estimates of vaccine efficacy (OR 1) based on the 20 cohort studies are:
56% (CI: 39%, 68%) for preventing respiratory illnes,
53% (CI: 35%, 66%) for preventing pneumonia,
48% (CI: 28%, 65%) for preventing hospitalisation, and
68% (CI: 56%, 76%) for preventing mortality.
Vaccine efficacy in relation to the epidemic strain were expressed as ORs. Respiratory illness: same strain 0.51 (CI: 0.36, 0.73); drift strain 0.34 (CI: 0.20, 0.58).
Pneumonia: same strain 0.41 (CI: 0.26, 0.65); drift strain 0.32 (CI: 0.12, 0.83).
Hospitalisation: same strain 0.44 (CI: 0.28, 0.68); drift strain 0.30 (CI: 0.08, 1.07).
Mortality: same strain 0.31 (CI: 0.21, 0.46); drift strain 0.21 (CI: 0.10, 0.43)
Results from the case-control studies and the randomised double-blind placebo-controlled trial were not statistically combined; the authors simply stated the results of each study. In the case-control studies, vaccine efficacy ranged from:
32% to 45% for preventing hospitalisation for pneumonia,
31% to 65% for preventing mortality from pneumonia and influenza,
43% to 50% for preventing hospital deaths from all respiratory illnesses, and
27% to 30% for preventing deaths from all causes.
In the randomised double-blind placebo-controlled trial the vaccine was found to have a 50% or greater reduction in influenza-related illness.