Identification

• The major portions of the flu shot are viruses that have been deadened or diluted in strength. By exposing the body to these weakened viruses, the body can build up immunity to the virus if exposed to it during flu season. Therefore, deciding which viruses to include in the flu vaccine begins with pinpointing which strains have been prevalent in flu cases from around the world. To do this, the World Health Organization (WHO) collects samples, and researchers then choose three viruses for inclusion in the vaccine.
  
  

Growing the Vaccines

• Discovering the presence of viruses growing in hen's eggs in the early 1900s sparked the inspiration for creating the first flu vaccines. Some of these principles are carried forth today, as the viral ingredients for the vaccine are grown in hen's eggs. The virus is first bred in the laboratory, then injected into hen's eggs where the viruses are able to multiply.
  
  

Reaching the Proper Concentration

• One of the keys to the success of the flu vaccine is the ability to dilute the viruses enough to where the vaccine is not too strong, yet is effective. This process is one of the final steps before distribution, as the viruses are purified with chemicals such as formaldehyde. This ensures the vaccine is sterile enough to be mixed for injection.
  
  

Preservatives

• In addition to the viruses contained in a flu shot, the vaccine contains a preservative known as thimserosal. While this is present only in trace amounts in the vaccine, it is the ingredient that comes most under fire, as it contains 49 percent mercury, according to the Centers for Disease Control and Prevention (CDC). However, the CDC assures Americans that the vaccine is safe, even for pregnant women to receive.
  
  

Vaccine Complications

• Because the flu vaccine contains materials that were grown in hen's eggs, those who are allergic to poultry should not receive the vaccine. While researchers are currently in development of a vaccine that is grown using mammal cells, these vaccines have not yet been approved for American use.