Influenza, often called the flu, is a contagious respiratory illness that spreads every year around the world.
It is caused by influenza viruses that infect the nose, throat, and sometimes the lungs.
The flu can lead to mild symptoms such as fever, cough, sore throat, and tiredness, but in some cases it can cause serious illness, hospitalization, or even death.
Young children, older adults, pregnant women, and people with certain health conditions are especially at risk of severe complications.
Because influenza viruses change frequently, health experts recommend that people receive a flu vaccine every year. The vaccine helps the body prepare to fight the virus if a person becomes exposed to it later in the season.
However, the protection offered by the vaccine can vary from year to year. This is mainly because scientists must predict which influenza strains will circulate months before the flu season begins.
New research from the U.S. Centers for Disease Control and Prevention (CDC) shows that the influenza vaccine for the 2025–2026 season appears to be less effective than vaccines used in some recent years.
Even so, the vaccine still provides important protection, especially for children and teenagers. The findings were published in the March 12 issue of the CDC’s Morbidity and Mortality Weekly Report.
The study was led by Patrick Maloney, Ph.D., a researcher at the CDC in Atlanta. His team analyzed data from several national research networks that track how well flu vaccines work. These networks study people who visit outpatient clinics for respiratory illnesses as well as patients who are hospitalized with flu-related infections.
Researchers examined information from people who became sick with acute respiratory illnesses during the current influenza season. By comparing vaccinated individuals with those who were not vaccinated, the scientists were able to estimate how well the flu vaccine prevented illness and severe outcomes.
The results showed that the vaccine provided moderate protection, particularly among younger people. Among children and adolescents younger than 18 years old, the vaccine reduced the risk of flu-related outpatient visits by about 38% to 41%. In addition, it reduced the risk of flu-related hospitalizations by about 41%.
For adults aged 18 years and older, the level of protection was somewhat lower. In this group, vaccine effectiveness ranged from about 22% to 34% in preventing outpatient visits related to influenza. The vaccine reduced the risk of hospitalization by about 30% among adults.
The researchers also looked at how well the vaccine worked against specific types of influenza viruses. Influenza viruses are commonly grouped into types such as influenza A and influenza B. Within these groups, there are several subtypes that can circulate during a flu season.
For influenza A infections, vaccine effectiveness among children and adolescents ranged from about 37% in preventing outpatient visits to about 42% in preventing hospitalization. Among adults, the vaccine provided protection ranging from 30% against hospitalization to about 34% against outpatient illness.
One important subtype of influenza A is called H3N2, which has been responsible for many severe flu seasons in the past. In this study, the vaccine reduced the risk of outpatient illness caused by influenza A(H3N2) by about 35% among children and adolescents. It also reduced the risk of hospitalization from this subtype by about 38%.
The researchers also measured protection against influenza B viruses. In adults, the vaccine reduced the risk of influenza B outpatient illness by about 63%. Among children and adolescents, protection ranged from about 45% to 71%.
Although these numbers are lower than in some recent seasons, health experts emphasize that the flu vaccine still provides valuable protection. Even moderate protection can significantly reduce the number of severe cases.
According to the study authors, influenza vaccination has prevented thousands of hospitalizations and deaths in the United States over the years. The vaccine can also reduce how severe the illness becomes in people who do get infected.
The findings highlight an important point about influenza prevention. Vaccines do not need to be perfect to provide public health benefits. Even when effectiveness is lower, vaccination helps slow the spread of the virus and protects vulnerable people in the community.
Another important benefit is that vaccinated individuals who still get sick often experience milder symptoms and recover faster than those who are not vaccinated. This can reduce pressure on hospitals and healthcare systems during busy flu seasons.
The study also reminds scientists that influenza viruses constantly evolve. Because the virus changes so often, researchers must continue to monitor circulating strains and update vaccines each year. Improving vaccine design and prediction methods remains an important goal for future research.
Overall, the results suggest that the 2025–2026 influenza vaccine is still an important tool for protecting public health, even though its effectiveness is somewhat lower this season. Vaccination remains one of the safest and most reliable ways to reduce the risk of severe flu illness, especially for children, older adults, and people with chronic health conditions.
The study was published in the Morbidity and Mortality Weekly Report from the U.S. Centers for Disease Control and Prevention.
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