We think of vaccines as antigens, but in truth vaccines contain additional ingredients that have functions ranging from stabilizing the antigens to preventing them from sticking to the vial. The nature of these substances varies widely and includes the following: proteins; sodium and potassium salts; buffers; sugars; antibiotics; preservatives; amino acids; inactivating agents; and detergents. Some of these are added purposefully in measurable, albeit small, quantities—gelatin or sucrose, for example, added as stabilizers. Others “leak through” from the manufacturing process and are present in only trace amounts—formaldehyde and sodium deoxycholate, for example, used to inactivate and disrupt viruses.
Some vaccine ingredients can trigger allergic reactions in sensitized individuals. A classic example is the residual egg protein are egg-allergic. Other examples include the gelatin and neomycin in MMR. To some extent, these reactions can be avoided through proper screening (see Chapter 4: Vaccine Practice—Screening).
None of the other ingredients in vaccines have been demonstrated to be harmful in the quantities used. For example, influenza vaccines may contain at most 100mcg of residual formaldehyde; for a 10kg child, that would amount to 10mcg/kg of exposure on one day. The (oral) reference dose for formaldehyde, which is an estimate of daily exposure that is likely to be without appreciable risk of deleterious effects during a person’s lifetime, is 200mcg/kg. Moreover, a 10kg child would be expected to have more than 2000mcg of formaldehyde circulating in blood at any given time, the result of normal biosynthetic processes.
—Marshall, Gary S. “Addressing Concerns About Vaccines.” The Vaccine Handbook: A Practical Guide for Clinicians. 3rd ed. New York: Professional Communications, Inc., 2010. 213-214. Print.