With the flu season now well underway in the Northern Hemisphere, you might be wondering about the effectiveness of this season’s flu vaccine.
Each flu season, the Centers for Diseases Control and Prevention (CDC) conducts studies to see how well flu vaccines for that season are working. The CDC has now released interim estimates of the 2024–2025 flu vaccine effectiveness based on data collected from October 2024 through February 2025.
According to the newly released CDC report, vaccine effectiveness against any influenza in children and adolescents was between 32% and 60% in outpatient settings, and between 63% and 78% against influenza-associated hospitalization. For adults aged 18 and older, vaccine effectiveness was 36% to 54% in outpatient settings and 41% to 55% against hospitalization.
Based on last year's Southern Hemisphere data, experts estimated that the Northern Hemisphere flu vaccine would reduce the risk of hospitalization due to the flu by around 35%, once all data is collected for the 2024–2025 flu season.
What Strains Do 2024–2025 Flu Vaccines Protect Against?
All flu vaccines for the 2024–2025 season are trivalent, meaning they are designed to protect against three flu strains. These include an influenza A(H1N1) virus, an influenza A(H3N2) virus, and an influenza B/Victoria virus.
These strains were selected based on what researchers anticipate will be the most likely to spread and cause illness during the flu season. The CDC reports that most influenza viruses detected in the United States this season have been influenza A viruses, which made up 97% of flu virus specimens tested. Of these, about 52% were identified as influenza A(H3N2); 47% were A(H1N1)pdm09 viruses.
When the CDC recently tested the H3N2 strain against vaccine-targeted antibodies, about 50.9% of the samples were "well-recognized" by the antibodies. At the same time last year, the CDC reported that all H3N2 virus strains were “well-recognized” by vaccine virus antibodies.
Flu vaccine effectiveness for influenza A(H3N2) strains tends to be lower than for Influenza A(H1N1) and Influenza B viruses. This may be because Influenza A(H3N2) viruses usually undergo more genetic changes over time than Influenza A(H1) and Influenza B viruses. Thus, when an Influenza A(H3N2) virus component is used in a flu vaccine, the H3N2 virus in circulation may change in a way that affects how well the H3N2 vaccine will work against that virus strain.
“‘[As] a rule of thumb, H3N2 typically has a higher impact [than H1N1] on older adults and you see a lot more hospitalizations,’” noted Josh Petrie, associate research scientist at the Marshfield Clinic Research Institute, to CBS News.
Flu Vaccine Effectiveness Varies Each Year
It's important to understand that flu vaccine effectiveness can vary quite a bit from year to year. Over the past decade, effectiveness has ranged from a low of 19% to a high of 60%. During the 2023–2024 season, flu vaccines were estimated to be 42% effective.
Several factors influence effectiveness, including the similarity between flu strains in the vaccine and those that are circulating during the flu season that vaccine is used. Because the flu virus is constantly mutating (changing), scientists continuously monitor flu viruses worldwide to determine which strains will be likely to circulate in our upcoming flu season. These flu strains are then used in the new seasonal flu vaccines, which require several months for manufacturing.
The CDC’s Influenza Division tracks flu viruses around the world throughout the year, as well. It also studies blood serum to understand how the human immune system will respond to flu viruses. This serology work helps improve the candidate vaccine viruses are assessed and selected for use in flu vaccines in the next flu season.
Even a vaccine with lower effectiveness can mean the difference between a mild illness and a hospital stay. Studies have shown flu vaccination lowers the risk of flu-related hospitalization by 41% in young children and 82% in adults. Vaccinated individuals who still get sick with flu are also less likely to be admitted to the ICU.
