Sore throat. Stuffy nose. Fever, body aches and fatigue. As temperatures drop, cases of the flu have started to rise.
For many people, the immune system can clear those symptoms in about a week. But for others, flu can lead to severe illness, hospitalization or even death. Flu infections cause up to 710,000 hospitalizations and 51,000 deaths every year in the United States, according to the Centers for Disease Control and Prevention.
A new analysis by the C.D.C., which examined flu-related hospitalizations from 2010 to 2023, unpacks some of the factors that put people most at risk of severe health outcomes.
Age: Young children and older people are consistently hospitalized with the flu at the highest rates because their immune systems are less robust than those of older children or younger adults, meaning they are less capable of fending off infection.
There are two types of flu viruses that can wreak havoc every season: Type A and Type B. The highest hospitalization rates since 2010 occurred during flu seasons when influenza Type A viruses were predominant; the C.D.C. has said this is because older adults tend to be more affected by H3N2, a Type A virus.
Neurological disorders: Some children with neurological conditions such as epilepsy, cerebral palsy and A.D.H.D. may have trouble with muscle and lung function, and may have trouble coughing or clearing fluids from their airways. This can exacerbate flu symptoms or lead to pneumonia.
Lung diseases: Flu can set off asthma attacks in children with the condition, which causes chronic inflammation of the airways. It can also lead to pneumonia and other respiratory issues that can require hospitalization.
In adults with chronic obstructive pulmonary disease, who have an impaired immune response, the body can have a harder time clearing a bout of flu. An infection can also further inflame the airways, making it even more difficult for patients to breathe.
Pregnancy: Nearly 30 percent of reproductive-age women who were hospitalized with flu during the 13 flu seasons the C.D.C. examined were pregnant. Studies have shown that pregnancy hormones suppress the immune system, resulting in more severe illness and higher hospitalization rates. A flu infection during pregnancy also carries some risk to the fetus.
Obesity and chronic metabolic diseases: For both children and adults, obesity is a risk factor for severe illness from flu. That may be because of several different factors. Excess weight can make it more difficult to take the deep breaths needed to clear an infection from the lungs, for example, said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center.
Obesity is often also linked with other metabolic diseases, like diabetes and hypertension. In patients with diabetes, a bout of flu can make blood sugar levels harder to control. And high glucose levels are thought to affect white blood cell function, worsening infections and prolonging recovery times, said Dr. Susan Spratt, a professor of medicine in the division of endocrinology, metabolism and nutrition at Duke University School of Medicine.
Heart disease: About half of adults hospitalized with the flu have heart disease, according to the C.D.C. People with cardiovascular disease tend to be older adults with less robust immune systems, said Dr. Sadiya Khan, a cardiologist at Northwestern Medicine. Studies have shown that flu increases the risk of heart attack and stroke in people with cardiovascular disease, she said.
“People who have heart disease are less able to tolerate the stress of the infection itself,” she said.
High blood pressure: In the two most recent flu seasons, one-quarter of patients between the ages of 18 and 49 hospitalized with flu had high blood pressure. About three-quarters of hospitalized patients older than 65 had the condition. High blood pressure can stiffen or damage the arteries, taxing the circulatory system. For people with hypertension, Dr. Spratt said, a flu infection can put even greater strain on the heart.
Vaccination status: Flu vaccines have been shown to significantly reduce the risk of hospitalization in children and adults. A recent analysis found that vaccination reduced the rate of hospitalization by 34.5 percent this year in five South American countries, where the flu season occurs earlier in the year.
Yet flu vaccination has been dropping in the United States. Slightly less than half of Americans 6 months and older received a flu vaccine last year.
Inequities in flu vaccine uptake may partly explain racial and ethnic disparities seen in hospitalization rates. The C.D.C. found that Black patients of all ages were hospitalized at the highest rates across most of the seasons in its study. Black, Native American or Alaskan Native patients had between 1.5 and 3.5 times higher flu-related hospitalization rates than white patients, the study found.
Even if you are vaccinated, it’s important to watch out for symptoms that might warrant medical care, like difficulty breathing and chest pain. Experts also recommend seeking care if you have other flu symptoms, such as a fever or cough, that don’t go away on their own within a week.
“The vaccine is not a panacea,” Dr. Schaffner said. “It’s just less likely that you will need hospitalization.”
