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 most common and concerning strains expected to circulate. But how do experts predict which ones will dominate months in advance?

The Ever-Changing Flu Virus

Influenza is a fast-moving RNA virus that changes as it spreads. To be effective, the annual flu vaccine needs to match the circulating strains. 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 different genotypes and the specific viruses within them change due to mutation and reassortment.

• Mutation – As flu spreads, small genetic changes (mutations) accumulate, leading to new viral variants.
• Reassortment – Flu viruses have a segmented genome, meaning if two different strains infect the same host, they can swap genetic material – like when you shuffle a deck of cards- creating entirely new versions of the virus.

Because of these natural changes, 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 centers analyze thousands of flu virus samples, studying how they spread, mutate or change, and interact with existing immunity levels 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 strains might provide cross-protection against related viruses

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. It takes about nine months to produce and distribute flu vaccines 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 each year. It’s a remarkable example of global scientific collaboration in action.

Why is VIDO talking about the flu?

Did you know that several VIDO research scientists are currently working on influenza projects affecting poultry, swine and humans.