Flu patients who had an underlying medical condition (UMC) were more likely to be hospitalized for their infection than those without one—and in the case of congestive heart failure (CHF), the risk was fourfold, according to a US Centers for Disease Control and Prevention (CDC)-led study.

44% vaccinated against flu

For the retrospective study, published today in the Journal of Infectious Diseases, the investigators used electronic health records and billing data to estimate flu-related hospitalization rates by type and number of UMCs among adult patients at two healthcare systems in California and Minnesota during four respiratory virus seasons (2016-17 to 2019-20).

Among UMCs, the team included blood disorders, cerebrovascular disease, asthma, chronic obstructive pulmonary disease (COPD), CHF, ischemic heart disease, liver disease, diabetes, neurologic or musculoskeletal disorders, chronic kidney disease, and rheumatologic or autoimmune conditions. 

The median patient age during the first included flu season was 46 years, 55% were women, 78% were White, 44% were vaccinated against flu, and 42% had at least one UMC.

“Individuals with various underlying medical conditions (UMCs) such as heart disease, chronic obstructive pulmonary disease (COPD), or diabetes mellitus have higher risk for acute respiratory illness (ARI)- and influenza-associated hospitalization,” the researchers wrote.

Rate rose for each additional UMC

Of 870,888 adults, 1,403 were hospitalized for flu at least once (1.2 per 1,000 person-years). Relative to those without CHF, the rate of flu-related hospitalization was highest for patients with CHF (13.0 per 1,000 person-years; adjusted rate ratio [aRR], 4.2), followed by COPD (4.0) and blood disorders (3.6), compared with those without the specific UMC.

Among those with varying types and number of UMCs, if vaccination prevalence had been lower than observed, influenza-associated hospitalization rates would have been higher.

The rate also increased with each additional UMC, compared with those with no UMCs (aRR for 1, 3.4; 2, 8.1; 3, 14.2; and 4 or more, 24.0). The effect of UMCs on hospitalizations for flu was higher when not mediated by vaccination status; for those with four or more UMCs versus no UMCs, rates were about 60% higher.

“Among those with varying types and number of UMCs, if vaccination prevalence had been lower than observed, influenza-associated hospitalization rates would have been higher,” the study authors wrote. “These findings highlight the importance of preventive medical care and annual influenza vaccination in reducing influenza-associated hospitalizations, particularly for individuals at high-risk.”

The CDC’s Advisory Committee on Immunization Practices recommends flu vaccination for everyone aged 6 months and older and particularly for those with UMCs because of their increased risk of severe illness and complications.