The Centers for Disease Control conducts annual studies to estimate the effectiveness of flu vaccines. While there are variations in in the effectiveness of a vaccine in a given year, on average, "flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population," according to the CDC.
Variations due to age and underlying medical conditions impact the overall effectiveness of a given vaccine, as does the mix of viruses in circulation in the population in a given season. In the 2019-2020 season, the overall vaccine effectiveness across age groups for all vaccine types against Influenza A or B was 39%. Effectiveness was higher (42%) among people aged 50-64.