The two main types of influenza virus that infect people are influenza A and influenza B. Influenza A can be found in many different animals as well as humans, while influenza B only infects humans.

There are many subtypes of the influenza A virus, which are named after the proteins on their surface. The two most important proteins for the influenza virus are hemagglutinin (H) and neuraminidase (N). Hemagglutinin is a protein on the virus’ surface that acts like a key, allowing the virus to attach and enter host cells. Once inside a cell, the virus replicates and produces new virus particles. Neuraminidase is another surface protein that helps these newly formed viruses spread from infected cells.

There are 18 hemagglutinin subtypes, H1 to H18, and 11 neuraminidase subtypes N1 through N11. Influenza A viruses are named based on the combinations of H and N present on their surface, such as H1N1 or H3N2. However, not all combinations of H and N can infect humans, and they are not all spreading at the same time.

There are two main lineages of influenza B virus: Influenza B Yamagata and Influenza B Victoria. These viruses only infect humans and spread seasonally with influenza A. However, influenza B causes fewer cases of flu each year than influenza A. Interestingly, influenza B Yamagata has not been detected since 2020.

What is Antigenic Drift vs Antigenic Shift?

Antigenic drift is the gradual process by which the influenza virus changes over time. As the virus replicates, tiny changes can occur in the genes, including the parts of the virus recognized by the immune system. Over time, these changes can accumulate, making the virus appear increasingly different to the immune system. This is one of the reasons seasonal influenza vaccines need to be updated regularly and why you can get the flu more than once.

Antigenic shift is a sudden, major change in the influenza virus that results in the creation of a substantially different strain. This can occur when two different strains of the influenza virus infect the same host and exchange genetic material, producing a different virus. Because this change is so sudden, it is likely the immune system has little memory, if any, of this virus’s combination of H and N. Antigenic shift has been responsible for the emergence of new influenza A viruses that can spread from animals to people and sometimes lead to pandemics.

While antigenic drift causes small, incremental changes, antigenic shift produces dramatic changes that can sometimes lead to outbreaks or pandemics if the new virus spreads efficiently from person to person.

Why are Flu Vaccines Recommended Every Year?

Flu vaccines contain harmless pieces of proteins from the influenza virus. These pieces of viral protein cannot cause disease but can still trigger an immune response. Using these pieces of influenza virus, the immune system learns how to fight the virus and creates antibodies to combat the virus during future encounters.

However, because of antigenic drift, the influenza virus can often change from year to year so that the immune system does not recognize it as well anymore. Antibody levels from previous vaccination can also decline over time, which is another reason annual vaccination is recommended. In the United States, flu vaccines are updated annually based on which flu strains are spreading across the globe and are most likely to cause disease that upcoming fall.

What is a Universal Flu Vaccine?

To address the need to both make and administer new flu vaccines each year, some researchers are focused on developing a universal flu vaccine. A universal flu vaccine would provide strong, long-term protection against multiple types of influenza virus, rather than a seasonal flu vaccine that needs to be updated each year.

Researchers are also investigating if mRNA vaccines could be used as flu vaccines. mRNA vaccines can be updated more quickly than many current flu vaccines making them a promising option for future seasonal influenza vaccines and may also help researchers develop future universal flu vaccines.

To learn more about the influenza vaccine for your own health, talk to your doctor or local health clinic.