Efficacy of influenza vaccines in healthy adults

This systematic review evaluates the efficacy and safety of influenza vaccines in healthy adults, including pregnant women, incorporating data from 52 clinical trials involving over 80,000 participants. Inactivated influenza vaccines significantly reduce the incidence of influenza from 2.3% to 0.9%. To prevent one case of influenza, 71 individuals need to be vaccinated. The vaccines also reduce influenza-like illness (ILI) from 21.5% to 18.1%. For ILI, the number needed to vaccinate (NNV) is 29. There is a slight reduction in hospitalization risk, from 14.7% to 14.1%, though the evidence is of low certainty.

For pregnant women clinical trial show a 50% efficacy against influenza in mothers and a 49% efficacy in infants up to 24 weeks. Vaccination may slightly increase the incidence of fever from 1.5% to 2.3%. The review found no significant effect on abortion or neonatal death based on observational data.

In conclusion, inactivated parenteral influenza vaccines moderately reduce the incidence of influenza and ILI in healthy adults, with a small effect on hospitalization and working days lost. The impact on pregnant women and their newborns is less pronounced. Despite some increase in mild adverse events, there is no significant evidence linking vaccination to serious adverse outcomes.