Vaccines have modest impact on influenza in healthy adults

Healthy adults who received inactivated parenteral influenza vaccine rather than no vaccine probably had a 1% lower risk of experiencing influenza over a single influenza season (2.3% versus 1%, moderate-certainty evidence) and probably had a 3.4% lower risk of experiencing influenza-like illness (ILI; 21.5% versus 18.1%, moderate-certainty evidence). The numbers needed to vaccinate (NNVs) for influenza and ILI were 71 and 29, respectively, reflecting high rates of ILI in the control groups for many of the trials. The NNV of 29 conceals variation in the absolute reduction in ILI for low and high-risk groups, and the degree of benefit may vary at least in part due to inconsistent symptom classification.