The flu virus mutates frequently, so flu shot components change seasonally. Scientists can never be certain which flu strains will affect a given geographic region. To combat this, vaccine manufacturers use several different flu strains in the antigen, the main component of a flu shot. Scientists working for the Epidemiology and Prevention Branch of the U.S. Centers for Disease Control's Influenza Division track the flu virus, its impact on the general population and any antigenic shifts, or virus mutations. The CDC varies flu shot components based on any antigenic shifts.
Antigen
The antigen is comprised of inactive flu virus strains. There are three forms of the flu virus, A, B and C. Type A is further divided into 16 subtypes. Type B is not broken down into subtypes, but it is differentiated by different strains. According to the CDC, type C infections are generally confined to respiratory symptoms, and do not result in a pandemic. Flu shots contain inactive strains of type A and B flu viruses but not type C.
Thimerosal
Although it has been used as a preservative since the 1930s, thimerosal is a highly controversial flu shot component. Many people question the toxicity of the chemical, which is comprised of 49 percent ethylmercury. In a January 2006 article, Dr. John Clements wrote that without thimerosal, especially in poor countries, vaccines would succumb to dangerous bacteria, potentially endangering the recipient. In the United States, thimerosal-free flu vaccines are available in limited quantities, but are generally more expensive than the alternative.
Adjuvant
Adjuvants are routinely used in European flu shots and were seriously considered in 2009 by the U.S. Food and Drug Administration to combat swine flu. The FDA ultimately rejected using adjuvants because the swine flu vaccine produced a higher than anticipated immune response in patients. Adjuvants boost the body's absorption of the flu shot and increase the immune system response to the actual flu virus. Aluminum gels and salts such as aluminum potassium sulfate and aluminum hydroxide are routinely used as adjuvants.
Inactive Ingredients
There are a number of inactive flu shot components. A saline solution is used to dilute the flu shot to its proper consistency. Stabilizers, such as gelatin, keep the vaccine in its original condition, even in varying temperatures. There may also be trace components from the manufacturing process.
References
- CDC: Types of Influenza Viruses
- Bloomberg: Swine Flu Shot May Rely on Emergency Use of Additives
- NPR: What You Need to Know About Swine Flu Vacccine
- "Informa"; When Science is Not Enough--A Risk/Benefit Profile of Thiomersal-Containing Vaccines; C. John Clements, M.D.; January 2006
