Vaccines contain a number of preservatives or additives. Most of them are in such minuscule amounts as to be considered harmless. For example, the product insert of the injectable polio vaccine states that formaldehyde comprises a maximum of 0.02 percent per 0.5ml dose. If you do the math, that works out to four ten-thousandths of a milliliter, which is a tiny amount indeed. The vaccine insert also describes how antibiotics are used in the manufacturing, but are processed out and remain at the level of nanograms. Just for perspective, there are one billion nanograms in a gram and there are thirty grams in an ounce.
Other ingredients in vaccines that worry some parents are the animal or human tissues used in the production process. Many vaccines are grown on animal cells, such as chicken eggs or monkey kidney cells. Between 1955 and 1963, some manufacturing lots of the polio vaccine were contaminated with a monkey virus: the simian virus 40, or SV40. SV40 has recently been found in several human cancers, including lymphoma. It is not clear if this virus is the causative agent of the cancer or is merely an innocent bystander. It is clear that while some of the people with SV40 in their cancer received the polio vaccine in question, many people with SV40 in their cancer did not receive that vaccine. So if SV40 causes cancer, there must be a way of getting the virus other than the contaminated polio virus of the 1950s and 1960s.
Unfortunately, we don’t know if the SV40 contamination was significant. We do know that it is an example of the possibility of unknown side effects from vaccines. Although the current cell lines used for vaccines are screened for known infectious processes, some parents worry about infections that have not yet been discovered.
Our response to this concern is that so far there is no evidence that this is a problem and that the manufacturers are doing the best they can to limit infectious organisms. While this is not conclusive by any means, it is most likely a small risk compared to the known benefits of the vaccine.