How Are Flu Vaccine Strains Chosen Each Year?
Every year, scientists work behind the scenes to select the influenza (flu) virus strains that will go into the next seasonal flu vaccine. Since flu viruses are constantly evolving, vaccines must be updated annually to match the virus strains expected to circulate. But how do experts predict which ones will dominate months in advance?
The constantly evolving flu virus
Flu viruses spread between people when an infected person releases virus laden droplets into the air or has physical contact with others. Coughing, sneezing and talking in close proximity are prime examples of how the virus spreads.
Flu viruses are in a constant “arms race” with our immune system. When someone recovers from infection, their immune system develops protection to the specific virus that infected them. However, flu viruses gradually mutate , allowing them to evade existing immunity and potentially infect people again by next flu season.
This is why flu vaccines need to be updated and administered every year. To provide the best protection, vaccines must closely match the strains expected to circulate.
Currently, the three main circulating viruses are H1N1 and H3N2 from influenza A and the Victoria lineage from influenza B. Within each subtype, there are many strains that are subtly different from each other and affecting how well our immune systems recognize them.
Because flu viruses are constantly changing, experts must continuously track flu virus evolution to ensure vaccines remain effective.
The Global Flu Surveillance Network
Flu strain selection is a truly international effort. The process is coordinated by the World Health Organization (WHO) through its Global Influenza Surveillance and Response System (GISRS), which consists of 152 National Influenza Centers in 129 countries, who monitor flu activity year-round and five WHO Collaborating Centers, located in the United States, United Kingdom, Japan, China and Australia.
These centres analyze thousands of flu virus samples, monitoring how many people are infected with the different virus strains, how different strains are changing or mutating, and if these viruses can avoid existing immunity in the population.
How Vaccine Strains Are Selected
Twice a year, WHO convenes a panel of experts—including the directors of these Collaborating Centers, essential regulatory laboratories, representatives from public health agencies, and scientists and clinicians working on influenza viruses worldwide.
- February – Experts meet to recommend flu strains for the upcoming Northern Hemisphere flu season (which begins in the fall).
- September – A second meeting is held to select strains for the Southern Hemisphere (where flu season begins in the spring).
To make their recommendations, experts consider several factors:
- Which flu strains are currently causing illness worldwide
- How quickly those strains are spreading
- How well previous vaccines matched circulating viruses
- Whether new vaccine strains are needed to provide cross-protection against several virus strains within a subtype
Once WHO makes its recommendations, each country reviews the data and approves its own vaccine formulation based on national health priorities.
From Selection to Vaccine Production
After the flu strains are chosen, vaccine manufacturers get to work. The selected viruses are adapted for large-scale production. Producing and distributing millions of flu vaccine doses can take up to nine months, which is why scientists must select the strains well before the next flu season begins.
Why This Process Matters
Flu vaccines don’t prevent every infection, but they significantly reduce the risk of severe illness, hospitalization, and death. Because flu is unpredictable, some years the vaccine is a better match than others. However, even in seasons where the match isn’t perfect, flu shots still provide meaningful protection by reducing complications, and slowing virus spread.
By continuously monitoring flu activity and adapting vaccines accordingly, public health experts help protect millions of people worldwide from severe flu-related illness. It’s a remarkable example of global scientific collaboration in action.
Why is VIDO talking about the flu?
VIDO scientists study influenza viruses affecting people and animals, including poultry and swine. Their work contributes to a better understanding of how flu viruses spread, evolve and cause disease, helping inform the development of vaccines and other medical countermeasures.
This article was originally written by Dr. Angela Rasmussen and Candace Lamb, Feb. 24, 2025 revised for consistency with current recommendations by Dr. Troy Sutton on Oct 13, 2026.