According to the Centers for Disease Control and Prevention 5 to 20 percent of Americans are infected by influenza viruses each year. Influenza produces a spectrum of disease, ranging from mild, self-limited respiratory disease to life-threatening pneumonia. Getting an annual flu shot can significantly reduce your risk of getting the flu.
Inactivated Influenza Virus
The seasonal flu shot is a "trivalent" vaccine, which means it contains three different inactivated influenza viruses. Although the individual strains vary each year, the types of strains selected for the vaccine usually consist of an influenza A strain, an H1N1 strain (not the 2009 H1N1 strain) and an influenza B strain. For the 2009-2010 flu season, the Food and Drug Administration also approved a separate "monovalent," or single strain, vaccine designed exclusively to combat the pandemic H1N1 strain.
Growth Medium
The influenza virus is grown in fertilized chicken eggs, and trace amounts of egg white protein (ovalbumin) may remain in the vaccine. All but two manufacturers (Sanofi Pasteur and ID Biomedical) use antibiotics to prevent contamination of the chicken eggs with bacteria. Common antibiotics used include kanamicin, gentamicin, neomycin and polymixin B. People with egg and antibiotic allergies should share this information with their health care provider prior to receiving the flu shot.
Purifying Agents
Once the virus has grown in chicken eggs, it must be recovered. This step involves the addition of chemical detergents. Some examples of the detergents used include sodium taurodeoxycholate, polyethylene glycol p-isooctylphenyl ether and octoxynol-10. These are usually present in concentrations measured in a unit called "parts per million." Each manufacturer specifies the type and maximum concentration of the chemicals used in its brand.
Inactivating Agents
The influenza viruses used in the flu shot are inactivated, which means that they are no longer capable of causing infections. The reason for this terminology is that viruses are technically not "alive," so they cannot be "killed," as is the case for bacteria. Once the virus has been purified, flu shot manufacturers add beta-propionolactone or formaldehyde derivatives to perform this step. Each manufacturer's product literature specifies the exact agent used and the maximum concentration present in the final dose.
Buffer Solution
Once the virus has been inactivated, it is suspended in a small amount of liquid known as a buffer solution, or "vehicle." The flu shot buffer solution consists of sterile phosphate-buffered salt solutions with a pH and concentration similar to the fluid that surrounds the cells in the human body. Other ingredients of the buffer solution may include gelatin, arginine and sucrose. Gelatin from pigs is used as a stabilizer. Arginine is an amino acid component of protein. Sucrose is common table sugar.
Preservatives
All FDA-approved multi-dose vials of seasonal and H1N1 vaccine contain thimerisol, a mercury-containing compound used a preservative. A flu shot dispensed from a multi-dose vial contains about 25 micrograms (0.000025 g) of mercury as thimerisol. The manufacturer's product insert will always confirm the presence and quantity of mercury for a specific brand of flu shot.
