The flu shot is necessary every year for two main reasons. First, flu viruses constantly change, and the vaccine is updated annually to match the strains predicted to be most common. Second, the immune protection from the vaccine declines over time. As for effectiveness, it varies each season based on the match between the vaccine and circulating viruses, but it typically reduces the risk of illness by 40-60%. Even when it doesn't prevent the flu entirely, it often makes the illness much milder.