How Are Seasonal Flu Vaccines Made?

Each year, a new flu shot is developed to try to combat the strains of the virus expected to wreak havoc that season. But who decides which viruses make it into the vaccine? What new technologies are being used to produce that vaccine more efficiently?

Sabrina Stierwalt, PhD
5-minute read
Episode #174

Coughing, chills, runny nose, and fatigue mean it’s that time of year again—the time when we spread holiday cheer and, well, the flu. Here at QDT we know what the flu is, how it’s different than the common cold, who is more susceptible to flu symptoms, and how to avoid getting and spreading the flu (like getting a flu shot!). You may also know that the flu shot changes each year, but how is the vaccine made? Who decides what strains of flu go into the vaccine each year, and how do they make that choice?

What Goes into a Flu Vaccine?

All year long, 142 national influenza centers in 113 different countries collect data on the flu viruses impacting the world’s population. Specifically, they monitor which strains of the virus are making people sick, how efficiently those strains are spreading, and how well previous vaccines have worked to combat their targeted viruses.

All of these smaller centers then pass the results from their wide-reaching investigations to one of five World Health Organization Collaborating Centers for Reference and Research on Influenza: the Centers for Disease Control and Prevention (aka the CDC) in Atlanta, Georgia; the National Institute for Medical Research in London, UK; the Victoria Infectious Diseases Reference Laboratory in Melbourne, Australia; the National Institute for Infectious Diseases in Tokyo, Japan; and the National Institute for Viral Disease Control and Prevention in Beijing, China.

Scientists at each of the five main centers then gather and analyze the data together to identify new flu strains and to determine which strains of the virus are most likely to spread and cause illness in the upcoming flu season. Consultants from each center then meet twice each year. They meet in February to determine the recommended composition for the yearly flu vaccine to be produced in the northern hemisphere for the upcoming flu season, and they also meet in September to make the same decision for future patients in the southern hemisphere.

The virologic surveillance data used to help make their decision are reported through several different channels. Clinical laboratories and hospitals monitor where and when flu cases occur, as well as what strains are detected and what kinds of patients are affected.  Patients who have flu-like symptoms with no other obvious cause but who still do not have officially confirmed cases of the flu are also monitored and recorded. Any changes in the strains of flu themselves, as well as the geographic reach of each strain, are also tracked. To see just how the flu is spreading in your home state, the Epidemiology and Prevention Branch in the Influenza Division at the Centers for Disease Control in the US, produces an interactive report of their findings.

Another factor that can determine which strains of the flu will be included in the current flu shot is the ability to produce a working vaccine against that particular strain. Every vaccine must be thoroughly tested and approved by the FDA before it is made available to the public. If for any reason the production process is particularly slow for a given strain, that strain will not be included in the World Health Organization’s list.

Ultimately, however, the World Health Organization only recommends which viruses go into the flu vaccine each year. Individual countries make the final decision on whether or not to follow that recommendation. In the United States, the US Food and Drug Administration (the FDA) has the final say. To see the make up of the 2015-2016 flu vaccine, check out the CDC’s website.

How Are Flu Vaccines Manufactured?

In the US, once the FDA has determined the flu vaccine concoction for the year, all vaccines are then made by manufacturers in the private sector. Private companies can have their own slight variations in the production technologies that they use, but flu shots are most commonly made using fertilized eggs.

A research center, usually the CDC, provides a manufacturer with flu viruses that have been grown in chicken eggs. The manufacturer then injects those viruses into other, fertilized chicken eggs where the are incubated for several days so that the virus can replicate. The fluid containing the virus is then harvested and the virus is killed. (Note that for the nasal spray, an alternative to the flu shot, the virus is merely weakened and not killed entirely.)


About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.

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