stanford medicine



What is
a vaccine?

Immunization demystified

The idea behind most vaccines is simple: Get the immune system ready to fight — without exposing anyone to active germs. But Google “vaccine ingredients” and up pops Web site after Web site raising concerns about formaldehyde, mercury and pathogens. So what’s really in that syringe?

The Root of the Vaccine: Altered Pathogens

Viruses, dead or alive: Dead or weakened live viruses teach your body to recognize and fight the real nasty version, should you cross paths.

Defanged toxins: Toxoids are deactivated versions of the toxins produced by infectious bacteria. Toxoids appear in vaccines that protect against diphtheria, tetanus and whooping cough.

Masters of disguise: Manufacturers some-times use just a part of the virus, and sometimes slip that virus part into a less dangerous organism. An example is the hepatitis B vaccine, made by culturing Saccharomyces cerevisiae, known in the kitchen as baker’s yeast. Vaccine makers then insert a gene into the harmless yeast that encodes for the surface proteins of hepatitis B. Then they purify the proteins and use these in the vaccine. When the immune system comes in contact with this sheep in wolf’s clothing, it dispatches the yeast proteins — but remembers hepatitis B. 

Boosters for the shot: adjuvants

While they don’t trigger antibody production on their own, when combined with antigens in vaccines, adjuvants ramp up the body’s immune response. In the United States, aluminum salts are the only approved adjuvants for human vaccines.

Leftovers: residuals from manufacturing

Proteins: Pathogens are often grown in chicken eggs or yeast colonies. Small amounts of egg or yeast proteins can escape purification and end up in the final vaccine.
Antibiotics: Antibiotics keep cultures clear of bacterial contaminants during the growth and manufacturing process.

Inactivating agents: Manufacturers use several chemicals to kill or weaken pathogens for use in vaccines. The most familiar (and notorious) is formaldehyde. Formaldehyde has a rap sheet, there’s no doubt. The Environmental Protection Agency recognizes the chemical as a probable human carcinogen if exposure is unusually high or prolonged, and inhalation can cause a whole range of symptoms from watery eyes, coughing and headache to asthma attacks. But researchers have found that the amounts in vaccines cause no harm. In fact, all humans carry small quantities of formaldehyde in their blood — it’s a byproduct of healthy metabolism. The amount normally in an infant’s circulation exceeds the amount in an infant vaccine dose by at least tenfold.

Supporting players: suspending fluid and stabilizers

Common stabilizers like sucrose, lactose, monosodium glutamate, gelatin and human serum albumin protect vaccines from cold, heat and spoilage.

Keep it fresh: preservatives

Vaccine preservatives include phenol, a disinfectant; 2-phenoxyethanol, a phenol derivative sometimes found in cosmetics; and that lightning rod for controversy, thimerosal. In 2001, the Centers for Disease Control and Prevention called for the removal of this mercury-containing compound from nearly all U.S. childhood vaccines amid concerns that mercury exposure could cause developmental problems, especially autism. But despite the removal of thimerosal from vaccines, U.S. rates of autism have continued to rise. Still, many anti-vaccine advocates see the precautionary removal as an admission of harm, so the controversy rages on.





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