Abstract
BACKGROUND: Chronic illnesses pose a major global health challenge with an estimated 1.56 billion people affected worldwide in 2025, and 85% of these being older adults facing at least one chronic condition. These patients are particularly vulnerable to severe influenza complications and higher mortality rates due to weakened immune responses; in addition, vaccination rates in China remain significantly lower than those in developed nations. METHODS: This review examines how chronic conditions exacerbate influenza-related effects through immune dysfunction and metabolic imbalances, and how influenza infection worsens chronic diseases by triggering inflammation, suppressing immunity, and causing secondary infections that lead to respiratory complications, cardiac complications, and blood sugar disturbances. RESULTS: A bidirectional adverse interaction exists in which chronic illnesses increase influenza severity via poor immunity, while influenza accelerates chronic disease progression (e.g., cardiac events and diabetic ketoacidosis). Vaccination reduces hospitalization by 32-52% in patients with lung disease and mortality by 16-46% in diabetic patients, with good safety. CONCLUSIONS: The findings emphasize the urgent need for improved vaccination strategies in patients with chronic diseases. Such strategies are crucial to reducing disease burden, enhancing clinical outcomes, and improving quality of life, while also providing critical evidence for the development of public health policies.