How Are Seasonal Flu Vaccines Made?

Editor's note: This article relates to the seasonal flu vaccine. It is unrelated to the currently ongoing process of developing a COVID-19 vaccine.

You may 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...

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