Flu can be dangerous. Study found hospitalization is threat for old and young.

Seasonal flu may not just peak with fever, cough and a sore throat. Many people, especially seniors and younger children, land in the hospital with severe illness, according to a new study.

More than 100,000 people are hospitalized and 4,900 people die from flu complications annually in the U.S. Vaccines, which target last year’s dominant flu strains, can help you avert serious illness or death.

The study published Tuesday in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report showed the risks seasonal flu still poses, especially to people who haven't been vaccinated.

“At every age, there are people who have no underlying medical conditions that are still at risk of hospitalization,” Dr. William Schaffner, a study co-author and professor of infectious diseases at Vanderbilt University Medical Center, told USA TODAY. “The influenza virus, in all of its guises, is a formidable virus that can take a healthy person and put them in the hospital within 48 hours.”

Data collected by the CDC's flu surveillance network from 2010 to 2023 found people 65 and older had the highest hospitalization rate, followed by children 4 and younger. Overall, these groups tend to be at greater risk from infectious diseases.

The study – done by researchers from the CDC and more than a dozen other institutes, health departments and universities – was based on a review of hospitalization records taken from several U.S. regions. The findings don't reflect the whole country.

People with severe flu cases were more likely to have underlying medical conditions. The most common underlying conditions among children were asthma, neurological disorders and obesity, the study found. Among adults, the most common conditions were hypertension, obesity, chronic metabolic disease, chronic lung disease and cardiovascular disease, which are all prevalent in the U.S.

The start of the COVID-19 pandemic marked a dramatic decline in flu hospitalizations. Schaffner, the study co-author, said vigilant social distancing and reduced interaction among children amid shuttered schools may have accounted for reductions in the spread of flu.

The 2022-23 season brought 64.4 hospitalizations per 100,000 people, indicating the figures returned to pre-pandemic levels. However, flu hospitalizations were still fewer than in the 2017-18 season, when doctors saw 102.9 hospitalizations per 100,000 people, the highest rate in the study period. ICU admissions, medical ventilation and in-hospital deaths persisted, showing that “influenza continues to cause severe morbidity and mortality,” the report said.

Black people, Native Americans and Alaska Natives were more likely to be hospitalized for flu. They also experienced higher rates of ICU admissions.

The study said lower vaccination rates in these populations likely contributed to them having higher hospitalization rates than white people. It noted that further observation of trends would help explain such disparities and enable prevention efforts in communities of color.

The study highlights disturbing trends among several groups that deserve more attention, said Dr. Siobhan Wescott, the Dr. Susan and Susette La Flesche Professor of American Indian Health at the University of Nebraska Medical Center. Misinformation and an unwillingness to get vaccinated contribute to increased hospitalizations, she added.

“They’re giving us a lot to consider,” said Wescott, who was not affiliated with the study. “We have to start working on immunization rates and comorbidities, all in an Indigenous way for our population.”

The study also found the use of antiviral medications declined among patients from a high of 90.2% in 2018-19 to less than 80% in 2022-23, with more pronounced decreases in antiviral use among children. These medications, such as Tamiflu or Relenza, can reduce severe forms of disease and improve outcomes in hospital settings. The drop is a “concerning trend and highlights missed opportunities” to prevent influenza-related complications, the study says.

Hospitalized adults ages 18 to 49 were the least likely to receive seasonal flu vaccines. In 2014-15, that age group reached a peak of 31.1% vaccination, and, by the 2022-23 season, the average dropped to just 15.4%.

Hospitalized patients who had been vaccinated had a substantially reduced chance of being admitted to the ICU, and a 31% lower risk of dying compared with unvaccinated people.

The current flu vaccine may not be as effective as earlier iterations, a recent CDC study of the winter season in the Southern Hemisphere showed. However, it still provides protection and reduces the risk of hospitalization by about 34%, the study said.

Schaffner, the study co-author, said the best time to get vaccinated is in October and November at the start of flu season. He urged people who haven't made plans to take action, saying it’s not the time to wait.

Flu will predictably circulate each fall and winter, as will other respiratory illnesses such as COVID-19 and respiratory syncytial virus, or RSV, said Dr. Yvonne Maldonado, professor of pediatrics, global health and infectious diseases at Stanford Medicine. All can put people in the hospital, but vaccines can help prevent a person from ending up there.

"Why not be protected against it again?" she told USA TODAY. "We wear helmets, we stop at stop signs. We just want to think about prevention. It's one less thing to worry about."

The 2023-24 season was one of the worst in recent history based on the number of children who died from flu, CDC data found. The 199 children who died coincided with a drop in the vaccination rate.

(This story was updated with new information.)

This article originally appeared on USA TODAY: The flu hospitalizes thousands each year, CDC study finds