Younger, healthier people tend to have higher rates of protection from the vaccine because they have a higher antibody response to it. As you get older, your immune system is not as efficient as it once was and it's less likely to give as strong an immune response.
Also, Bridges says that in years when the flu vaccine is a strong match, the effectiveness rates for it in older adults and in those who are immunocompromised is closer to 60 percent
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