Why are older adults more susceptible to influenza?

As you age, your immune system gradually declines. This phenomenon is called immunosenescence. In practical terms, it means that the cells responsible for recognising and neutralising infections are produced less effectively and respond more slowly. As a result, the body builds up fewer protective immune cells after exposure to the influenza virus, giving the infection more opportunity to take hold.

This reduced resistance also affects how well vaccination works. Older adults respond less strongly to the flu vaccine than younger adults: fewer antibodies are produced and the response of the immune cells falls short. The vaccine does offer older adults some protection, but less than it does for younger people.

Beyond the flu itself, it is the complications that hit older adults hardest. The risk of a heart attack is more than six times higher than normal in the first week after an influenza infection, and that peak disappears after day seven. Pneumonia caused by a bacterial infection on top of the flu is the most common serious complication in hospitalised older adults aged 65 and over. Brain inflammation and disorders of consciousness have also been documented in smaller numbers, although those findings are based on a limited and retrospective study.

As a group, older adults also more often already have heart problems or other chronic conditions, which further increases the likelihood of those serious outcomes. This compounding effect -- a declining immune system combined with a vulnerable body -- explains why influenza leads to hospitalisation and death far more often in older adults than in younger people.