Table 2 Examples of evidence from systematic reviews comparing inactivated influenza vaccines with placebo or no intervention

Population Study design included in review Outcome No of participants Vaccine field efficacy or effectiveness* Children aged up to 23 months8 RCT Influenza 786 0.55 (0.18 to 1.69) Influenza-like illness — No data Children 6 years or more8 RCT Influenza 710 69%; 0.31 (0.22 to 0.45) Influenza-like illness 18 912 28%; 0.72 (0.66 to 0.78) Children up to 16 years8 RCT Transmission 123 1.68 (0.56 to 4.99) School absence 254 0.46 (0.17 to 1.22) Lower respiratory tract infection 136 0.30 (0.01 to 6.17) Admission to hospital 765 1.41 (0.62 to 3.24) Death — No data Healthy adults9 RCT Influenza 2411 67%; 0.33 (0.22 to 0.49) Influenza-like illness 5579 22%; 0.78 (0.67 to 0.91) Admission to hospital 5261 Relative risk fixed effects model 0.65 (0.34 to 1.22) Working days lost 5572 Weighted mean difference random effects model –0.12 (–0.24 to 0.00) Healthcare workers (to protect elderly patients in their care)10 Cluster RCT and cohort Influenza 752 0.87 (0.46 to 1.63) Lower respiratory tract infection 1059 0.70 (0.41 to 1.20) Death from pneumonia 1059 39%; 0.61 (0.38 to 0.98) All-cause mortality 2496 40%; 0.60 (0.50 to 0.73) Elderly people in the community; high circulation of influenza virus and good vaccine-antigen matching11 Cohort Influenza-like illness — No data Influenza 427 95%; 0.05 (0.01 to 0.37) Pneumonia — No data Death from influenza and pneumonia 163 391 0.87 (0.70 to 1.09) All-cause mortality (not adjusted for confounding) 300 332 41%; 0.59 (0.50 to 0.70) All-cause mortality (adjusted for confounding) 742 575 47%; odds ratio random effects model 0.53 (0.46 to 0.61) Elderly people in institutions; high circulation of influenza virus and good vaccine-antigen matching11 Cohort Influenza-like illness 5963 23%; 0.77 (0.64 to 0.94) Influenza 658 1.04 (0.43 to 2.51) Pneumonia 4482 46%; 0.54 (0.42 to 0.70) Death from influenza and pneumonia 6127 42%; 0.58 (0.41 to 0.83) All-cause mortality 305 60%; 0.40 (0.21 to 0.77) Patients with asthma12 RCT Influenza related exacerbation (early) 696 Risk difference fixed effects model 0.01 (–0.02 to 0.04) Patients with chronic obstructive pulmonary disease13 RCT Exacerbations (total number) 180 Weighted mean difference –0.37 (–0.64 to –0.11), P=0.006

RCT, randomised controlled trial.

^* Values are vaccine field efficacy or effectiveness (where available); relative risk random effects model (95% confidence intervals) unless stated otherwise. Relative risk reported when difference is not significant.

Placebo controlled comparison.