Young children, adults 65 and older, pregnant women, and people with certain chronic health conditions face the highest risk of serious flu complications. While anyone can catch the flu, these groups are far more likely to end up hospitalized or to develop life-threatening problems like pneumonia, organ failure, or secondary infections. Roughly 90% of flu-related deaths and 50 to 70% of flu-related hospitalizations occur in adults 65 and older alone.
Age: The Strongest Single Risk Factor
Age sits at both ends of the risk spectrum. Infants under 6 months have the highest hospitalization rates of any pediatric group, and children under 2 are considered high risk across the board. In a large study of hospitalized children with flu, the 6-month to under-5 age group had the highest complication rate at nearly 31%, with neurological problems like febrile seizures being the most common. Pulmonary complications, including pneumonia, were the second most frequent issue.
On the other end, risk climbs steadily after age 65. A CDC analysis found that both hospitalization rates and the likelihood of dying from flu increase with each decade past 65. The immune system naturally weakens with age, producing fewer and less effective infection-fighting cells. Older adults are also more likely to have underlying heart or lung disease that compounds the danger.
Heart Disease and the Risk of Heart Attack
People with heart conditions like congestive heart failure and coronary artery disease face a dual threat from the flu. The infection triggers widespread inflammation that can destabilize fatty plaques in arteries, making them more likely to rupture and cause a clot. A 2018 study found that people were six times more likely to have a heart attack in the week after a flu diagnosis. Even mild flu infections are associated with a twofold increase in acute cardiovascular events in older patients.
This risk isn’t limited to people who already know they have heart disease. The inflammatory surge from flu can reveal vulnerabilities in blood vessels that hadn’t caused symptoms before.
Diabetes and Blood Sugar Swings
People with diabetes are three times more likely to be hospitalized with flu, four times more likely to need intensive care, and twice as likely to die from the infection compared to people without underlying conditions. The reason goes beyond a generally weakened immune system.
In diabetes, blood sugar levels swing more dramatically and more often. These fluctuations cause oxidative stress, essentially a buildup of damaging molecules that injure the lining of blood vessels. During flu, the cells lining blood vessels play a central role in driving the “cytokine storm,” an overblown inflammatory response responsible for much of the lung damage in severe cases. When those cells are already stressed and inflamed from glucose swings, the inflammatory cascade hits harder. Oxidative stress also impairs the immune cells responsible for clearing the virus, particularly memory cells that should recognize and fight infections quickly.
Asthma and Chronic Lung Disease
Flu viruses can infect the lower airways directly, triggering a powerful inflammatory response that narrows already-compromised airways. In people with asthma, this means a sharp increase in bronchial hyperresponsiveness, the tendency of airways to spasm and constrict. The virus also ramps up mucus production, further blocking airflow.
For people with COPD, the stakes are even higher. Among COPD patients whose flare-ups were severe enough to require mechanical ventilation, a viral pathogen was detected in 46% of cases. The combination of damaged lung tissue, excess mucus, and impaired clearance mechanisms makes it extremely difficult for these patients to recover without intensive support.
Pregnancy and Postpartum
Pregnant women are at higher risk for severe flu throughout pregnancy and for up to two weeks after delivery. The immune system shifts during pregnancy to avoid rejecting the fetus, which leaves the body less equipped to fight respiratory viruses. The heart and lungs are also working harder to support the pregnancy, leaving less reserve to handle an infection.
A CDC study published in The Lancet Infectious Diseases found that flu during pregnancy was associated with reduced birthweight in full-term newborns and an increased risk of pregnancy loss after 13 weeks. Women who developed fever along with respiratory symptoms faced a higher risk of preterm birth.
Weakened Immune Systems
People whose immune systems are suppressed, whether from HIV, cancer, chemotherapy, organ transplant medications, or long-term steroid use, face prolonged and more severe flu infections. A healthy person typically clears the flu virus within about a week. Immunocompromised patients can shed the virus for months, giving it more time to cause damage and more opportunity to develop resistance to antiviral medications.
In one documented case, a child who had received a bone marrow transplant shed influenza from respiratory secretions for over a year and a half, despite aggressive antiviral treatment. During that time, the child developed secondary bacterial and fungal infections. While this is an extreme example, it illustrates how a compromised immune system fundamentally changes the course of a flu infection.
Neurological Conditions
Children and adults with neurological conditions face elevated flu risk for a mechanical reason that’s easy to overlook: many have trouble with muscle function, lung function, or the ability to cough, swallow, and clear fluids from their airways. This makes it harder to keep the lungs clear during any respiratory infection and raises the risk of pneumonia.
The list of qualifying conditions is broad. It includes cerebral palsy, epilepsy, stroke, muscular dystrophy, spinal cord injuries, intellectual disability, and moderate to severe developmental delays. The risk isn’t necessarily from the neurological condition interacting with the virus itself, but from the physical limitations that make it harder to manage the respiratory symptoms.
Obesity
The CDC lists a BMI of 40 or higher as a risk factor for severe flu. At that level, the mechanics of breathing are already compromised: excess tissue around the chest and abdomen restricts lung expansion, and baseline inflammation is typically elevated. These factors combine to make it harder for the body to mount an effective response to a respiratory infection while maintaining adequate oxygen levels.
Racial and Ethnic Disparities
Flu doesn’t hit all communities equally. Over more than a decade of surveillance data from 2009 through 2022, Black adults were hospitalized with flu at 1.8 times the rate of white adults. American Indian and Alaska Native adults were hospitalized at 1.3 times the rate, and Hispanic adults at 1.2 times the rate. These disparities reflect a combination of factors: higher rates of underlying conditions like diabetes and heart disease, less access to timely healthcare, lower vaccination coverage, and the cumulative effects of socioeconomic inequity on overall health.
Other Groups to Be Aware Of
Several additional conditions appear on the CDC’s high-risk list and are worth noting:
- Kidney and liver disorders impair the body’s ability to filter toxins and manage the metabolic demands of fighting infection.
- Blood disorders like sickle cell disease reduce oxygen delivery and can trigger painful crises during illness.
- Children under 19 on long-term aspirin therapy face the risk of Reye’s syndrome, a rare but serious condition affecting the brain and liver, if they contract the flu.
- Residents of nursing homes and long-term care facilities are at risk both because of their age and health status and because of the close-quarters living that accelerates viral spread.
Having more than one risk factor compounds the danger. A 65-year-old with diabetes and heart disease faces a meaningfully different level of risk than a 65-year-old who is otherwise healthy. The flu vaccine remains the most practical way to reduce severe outcomes across all of these groups.