With the flu season now starting 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. Last season, early estimates about flu vaccine effectiveness were published later in the flu season, in February, so we may not have solid data for the 2024-2025 season for a while. However, while it's impossible to predict with 100% certainty what is going to happen this flu season, we can look to the Southern Hemisphere's flu season for clues.
A recent report showed that various Southern Hemisphere countries—including Argentina, Brazil, Chile, Paraguay, and Uruguay—experienced a flu vaccine effectiveness of about 34.5% overall.
Based on this Southern Hemisphere data, experts estimate that the Northern Hemisphere flu vaccine will reduce the risk of hospitalization due to the flu by around 35%. That's lower than the 42% seen last season, but still within the typical range.
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. During the Southern Hemisphere's 2024 season, flu vaccines were most effective against A(H3N2) viruses (36.5% effective) and A(H1N1)pdm09 (37.1% effective). Effectiveness against influenza B viruses was not estimated due to low circulation.
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.
