The researchers calculated the healthcare burden of flu cases that would have occurred in the absence of vaccination based on factors such as illness and hospitalization rates during the flu season, vaccination coverage and vaccine effectiveness. Based on these data, Kostova and colleagues estimate that flu vaccines averted several million instances of illness and over 110,000 flu-related hospitalizations in the flu seasons of 2006 to 2011. The largest number of averted cases occurred during the most recent period studied, 2010-2011, when 5 million flu cases, 2.1 million medical visits and 40,400 hospitalizations were prevented by vaccination.

The U.S is the only country with universal influenza vaccine recommendations that suggest everyone aged 6 months and older should receive an annual dose of the vaccine. However, previous studies have not provided ways to reliably assess the number of flu cases or hospitalizations that are prevented by vaccination each year. Senior author on the study Joseph Bresee adds, ""These results confirm the value of influenza vaccination, but highlight the need for more people to get vaccinated and the imperative for vaccines with greater efficacy, especially in the elderly."