As the influenza season progresses, the Centers for Disease Control and Prevention (CDC) publishes estimates on the effectiveness of the influenza vaccines. Effectiveness is measured as the reduction in risk of influenza in the groups of people who get the vaccine, compared to those who do not. So far this season — the 2019-2020 season — estimates on effectiveness are somewhere between 45% overall and 55% in pediatric groups.
The influenza vaccine is not the best available vaccine, and much work needs to be done to improve it. That work is being done by researchers looking for a universal influenza vaccine — one that would not need to be given each year — as well as better ways to convince the public to get the yearly flu vaccine. If enough people get vaccinated against influenza, then that effectiveness shoots up due to community immunity.