How Influenza (Flu) Strains Are Selected For the Flu Vaccination

If you’re like me, you’ve always wondered what goes into making the flu vaccine each year. Who decides what strains are covered? Do we get the same vaccination as people in other countries?

How flu virus strains are selected for the vaccine:

The flu vaccine is made to protect against three to four different flu virus strains each year as there is not just one type of flu. Over 100 influenza centers in more than 100 countries monitor flu year round because the flu viruses are constantly changing. The flu centers study which viruses are making people sick, how far they are spreading, and how well the current season’s flu vaccination is working to protect people from those viruses.

Who decides which strains are actually in the vaccine and why does it change?

Viruses, like the flu, are constantly changing and adapting to be better at what they do: making more copies of themselves. With the flu virus constantly changing, the need for a vaccine covering different strains of the flu is important. The influenza centers mentioned above send samples of flu viruses to the World Health Organization (WHO) Centers in Atlanta, Georgia; London, United Kingdom; Melbourne, Australia; Tokyo, Japan; and Beijing, China. These WHO Centers review the information from the influenza centers twice per year and use the information to recommend which flu virus strains should be in the vaccination for the flu season based off of the more commonly seen strains and ones they think may be circulating during the upcoming flu season. The Food and Drug Administration (FDA) has the final say on which flu virus strains are actually put in the flu vaccination in the United States, based off of the WHO recommendation.

Different vaccines for different areas:

The WHO recommends different vaccines for the Northern Hemisphere and the Southern Hemisphere, but each country makes their own decision about which flu virus strains are actually contained in the flu vaccination.

How the vaccine is made:

Once the decision is made on which viruses should be in the upcoming flu vaccine, private manufacturers begin producing the vaccine. It takes at least six months to produce large quantities of vaccines to be distributed. That means to be ready for vaccinations in October, the manufacturers have to start growing the viruses by April or even sooner to allow distribution to the pharmacies, hospitals, and clinics that administer the vaccinations.

The FDA has approved three different ways to produce flu vaccinations: egg-based, cell-based, and recombinant flu vaccines.

Egg-based is the most common production type and is used to make both the inactivated vaccines (flu shot) and the live attenuated vaccine (nasal spray). The manufacturers receive sample vaccine viruses from either the CDC or a laboratory for the WHO and the viruses are injected into fertilized chicken eggs. The viruses are grown in the eggs, then taken from the eggs and killed, then purified to make the vaccination. For the nasal spray, the viruses are weakened before growing, and go through a different process to produce the nasal spray vaccine.

Producing cell-based flu vaccines is similar to the egg-based vaccines process, but involves replicating the viruses in different cells, then extracting the fluid from the cells and purifying and testing it.

Recombinant flu vaccines use certain proteins from the flu viruses that people usually have immune response to, and then grow that protein with a virus in cells. Then the protein is collected from the cells and purified to make a 100% egg-free flu vaccine.


Selecting Viruses for the Seasonal Flu Vaccine. Centers for Disease Control and Prevention. Published May 4, 2016. Accessed February 25, 2018.

How Flu (Flu) Vaccines Are Made. Centers for Disease Control and Prevention. Published November 7, 2016. Accessed February 25, 2018.