During years when the flu vaccine is not well matched to circulating influenza viruses, it is possible that little or no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications. However, even during years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on various factors like the characteristics of the person being vaccinated, what influenza viruses are circulating that season and even, potentially, which type of flu vaccine was used.