The influenza, or flu, vaccine you get at your doctor’s office or pharmacy is the year-round work of highly skilled microbiologists, epidemiologists, physicians & other public health experts.

Sound complicated? It is.

The U.S. Food & Drug Administration & the U.S. Department of Health & Human Services (HHS) are working toward developing new & better technologies for producing flu vaccines. As new strains of flu viruses emerge, the FDA works in close coordination with sister agencies, such as the Centers for Disease Control & Prevention (CDC) & the National Institutes of Health (NIH), encouraging manufacturers to develop vaccines that will successfully protect us from influenza disease, which can be a very serious illness.

The more diverse the influenza vaccine supply, the better we can respond to flu public health emergencies. All FDA-approved flu vaccines have been evaluated & determined to be safe & effective by the agency.

There is often more than one type of influenza virus circulating each season. So, vaccines are formulated to target four of the most likely influenza viruses to circulate & cause illness in the U.S. during the upcoming influenza season: two influenza A types (H1N1 & H3N2) & two types of influenza B. These are known as quadrivalent vaccines. Influenza B more commonly affects children & also causes more complications & death in children than adults.

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Influenza Vaccines & How They Are Made

The influenza vaccine works by triggering your immune system to produce antibodies that help the body prevent the flu.

Most of the U.S. influenza vaccine supply is made using an egg-based production process. In this method, manufacturers use fertilized eggs to grow flu viruses. After about six months of laboratory work & manufacturing, those viruses are incorporated into that season’s flu vaccine.

Although this method produces safe & effective vaccines, the FDA, HHS & manufacturers have been working on different technologies to increase the influenza vaccine supply, making sure to still maintain safety & effectiveness. Some technologies have the potential to produce these vaccines faster & more efficiently. For example, the FDA has approved influenza vaccines that do not use an egg-based production process for manufacturing.

In addition, the FDA has approved a high-dose & an adjuvanted vaccine specifically for people ages 65 & older, who typically bear the greatest burden of severe influenza disease & account for most of flu-related hospitalizations & deaths. Adjuvants are incorporated into some vaccine formulations to enhance the immune response of the vaccinated person.

Making the Flu Vaccine: A Year-Round Effort

The job of producing a new vaccine for the next flu season starts well before the current flu season ends. For the FDA, it’s a year-round initiative.

Flu viruses are constantly evolving. & the flu viruses that circulate & cause disease in people often change from one year to another. We need a new flu vaccine every year.

To that end, the FDA, World Health Organization (WHO), CDC & other partners collaborate by collecting & reviewing data on the circulating strains of influenza from around the world to identify those likely to cause the most illness in the upcoming flu season.

In late February/early March — well before the new flu season begins — an FDA advisory committee reviews data about which flu viruses have caused disease in the past year, how the viruses are changing, & disease trends so they can recommend the flu virus strains to include in the influenza vaccines for the U.S. in the upcoming flu season.

Once the strains are selected, vaccine producers begin the manufacturing process to include the newly selected flu strains in their FDA-approved vaccines. The different flu virus strains are combined to formulate the vaccine into standard dosages. The vaccine is then filled into vials, syringes &, for the nasal vaccine, sprayers.

Both egg-based & non-egg-based manufacturing methods for FDA-approved flu vaccines require high-tech processes & manufacturing facilities that have been inspected by the FDA. Vaccine manufacturers must submit applications to the FDA that include the new flu strains in their FDA-approved vaccines & updated prescribing information.

In its own laboratories, the FDA also produces materials that are critical for making the vaccine. These include generating candidate vaccine virus strains suitable for further vaccine manufacture & producing the critical potency reagents, which are materials needed to test the vaccines for potency & identity (to ensure standardization) before the FDA approves the new formulation of the approved seasonal influenza vaccines for U.S. distribution.

The FDA is also responsible for ensuring that released lots of influenza vaccines meet appropriate standards. Each vaccine undergoes quality control tests, including testing for sterility. Manufacturers submit the results of their testing, along with sample vials from each lot to the FDA for “lot release.”

The FDA typically begins releasing lots of flu vaccines in late summer. Lot release can continue into early fall. Once lots are released, manufacturers distribute the vaccine throughout the U.S. for use by the public.

A Flu Vaccine Is the Best Prevention

Flu seasons & severity are unpredictable. Annual vaccination is the best way to prevent the flu for people ages 6 months & older.

An annual immunization with flu vaccine is the most effective & safest way for most of us to reduce our risk of getting the flu & spreading it to others. When more people get vaccinated, it is less likely that the flu viruses will spread through a community, making us all healthier.