Influenza vaccination reduces the number of influenza virus infections but not the overall number of illness.

Influenza virus causes seasonal epidemics, resulting in 3–5 million severe cases and 250000–500000 deaths globally each year. Elderly persons, individuals with certain medical conditions and children aged <2 years have the highest risk for complications. Vaccination is an important tool to prevent infection and to reduce morbidity and mortality .

In summary, we show that influenza virus caused between 18.9% and 34.2% of Influenza-like Illness cases in community-dwelling older adults aged ≥60 years in 2 influenza seasons in the Netherlands, leaving the remainder caused by other pathogens. We also show that influenza vaccination was effective in reducing the incidence of influenza virus infections but did not reduce the Influenza-like Illness incidence, which may have important public health and healthcare consequences. Our data will also help to better inform the public what to expect from influenza vaccination and how it will not protect against all cases of ILI, popularly seen as “flu.”