How Flu Strains are Selected for the Seasonal Flu Vaccine Each Year
Mar 14, 2018
by Serese Marotta, Chief Operating Officer of Families Fighting Flu
The Centers for Disease Control and Prevention (CDC) recommends that everyone ages 6 months and older, with rare exception, get an annual flu vaccine. But did you ever wonder how the flu strains are selected for the seasonal vaccine every year?
A lot more goes into the decision than you might think!
Seasonal flu vaccines contain three (trivalent) or four (quadrivalent) flu strains. Because flu is a complex, dynamic virus that is constantly changing, there are more than 100 monitoring centers in over 100 countries located across the globe that monitor flu activity on a year-round basis to identify which flu strains are circulating.
These centers receive and test thousands of influenza virus samples from patients. They then send representative virus samples to five World Health Organization (WHO) Collaborating Centers for Reference and Research on Influenza, located in Atlanta, GA (i.e., the CDC); London, United Kingdom; Melbourne, Australia; Tokyo, Japan; and Beijing, China. The surveillance data gathered from these samples, along with other information, are used to make a recommendation on which flu strains should be included in the upcoming year’s seasonal flu vaccine.
Contrary to popular belief, the flu vaccine is not just based on last year’s flu viruses. Three general sources of information are considered in the selection of flu strains for the seasonal flu vaccine:
- Surveillance data represents information gathered from the influenza monitoring centers that collect virus samples from patients. Experts use this information to determine which flu strains are circulating and where.
- Laboratory data refers to antigenic characterization of the flu viruses in a laboratory, which simply means the identification of specific molecular structures on the influenza virus that are recognized by our immune systems and elicit an immune response. The antigen is the “invader” (i.e., in this case, the flu virus) that causes our immune systems to launch an attack through the formation of specific antibodies. Antibodies are what our bodies produce following flu vaccination so that it’s properly “armed and ready” to recognize and fight that specific flu virus if and when we’re exposed.
- Genetic characterization of flu viruses may also be considered in the selection of vaccine strains. This refers to “mapping” of the genetic codes that make up each flu strain, which allows the experts to monitor changes in circulating flu viruses.
- Data from clinical studies on vaccine effectiveness are also considered.
With this robust amount of data in hand, the WHO then meets twice per year to make a recommendation for flu vaccine strains for the upcoming season: once in February to recommend flu strains for the Northern Hemisphere seasonal flu vaccine, and again in September to recommend flu strains for the Southern Hemisphere seasonal flu vaccine. But it doesn’t stop there! Each country then considers the WHO recommendation, reviews the available information, and makes their own decision on which flu strains to include in their country’s seasonal flu vaccine.
In the U.S., once the WHO makes their recommendation for flu strains for the upcoming year’s seasonal flu vaccine, an advisory committee from the U.S. Food and Drug Administration (FDA) meets in February or March to review the WHO’s recommendation and supporting information and vote on the final selection of flu strains. The role of the FDA is an important one, because once the flu strains are selected, the FDA produces materials in their laboratories that are critical for actually producing the flu vaccines. For instance, the FDA provides vaccine manufacturers with the seed viruses and the potency reagents needed to ensure that flu vaccines made by one manufacturer are similar to those made by another. The FDA also conducts quality control measures by ensuring that batches (referred to as “lots”) of flu vaccines released by the manufacturers meet appropriate standards and reflect the correct genetic composition.
Following the selection of flu strains for the seasonal vaccine and receipt of the appropriate materials and information from the FDA, private sector manufacturers begin the process of making the vaccines. All flu vaccines in the U.S. contain the same flu strains, i.e., the flu vaccine available in New York contains the same three or four flu strains as the vaccine that’s available in California. And it’s important to remember that all flu strains (influenza A or B) can be potentially dangerous, regardless of an individual’s health status, and are capable of causing serious illness, hospitalization, or even death.
Influenza is a vaccine-preventable disease that has the ability to affect all of us around the world, which is why it remains such a pressing global public health issue. Seasonal flu vaccines may not be perfect, but given the complexity of flu viruses and their ability to change and mutate frequently, the U.S. does have a solid, scientifically-based approach for flu vaccine development. While much research and development is being done for a universal flu vaccine, the possibility of this technological advancement is still many years off. In the meantime, let’s not forget all the hard work and research that goes into helping to protect us with the currently available seasonal flu vaccines. And if you’re wondering “why bother” with a flu vaccine that may be substantially less than 100% effective, let’s remember that something is better than nothing, especially when it comes to your life or the life of a loved one.
More in-depth information on how flu strains are selected for the seasonal flu vaccine every year are available from the CDC and FDA.
About Families Fighting Flu: Families Fighting Flu (FFF) is a national, nonprofit, 501(c)(3) volunteer-based advocacy organization dedicated to protecting the lives of children and families by helping to increase annual influenza vaccination rates, especially among children 6 months and older and their families. Our members include families whose children have suffered serious medical complications or died from influenza, as well as healthcare practitioners and advocates committed to flu prevention. In honor of our children, we work to increase awareness about the seriousness of influenza and to reduce the number of hospitalizations and deaths caused by the flu each year.
Vaccine passports have quickly become the latest lightning rod in COVID-19 political discussions in the United States. Shortly after New York introduced their version — called the Excelsior Pass — states began to take...
Connecticut has removed all non-medical vaccine exemptions for children in schools and daycares. The state legislature’s vote came after three years of debate and record-breaking hours of public testimony. Connecticut now joins five other...