evidence: Vaccine ingredients can look unfamiliar. However, it is important to remember that many of the substances used in vaccines are found naturally in the body. For example, many vaccines contain salts based on sodium and potassium (see the section on 'Acidity regulators'), which are essential for life. People may think of formaldehyde as a man-made chemical, but in small quantities, it is also found naturally in the bloodstream. All vaccine ingredients are present in very small quantities, and there is no evidence that they cause harm in these amounts. The exception to this is the small number of people who may be severely allergic to a vaccine ingredient, even if it is present only in trace amounts (for example, egg proteins or antibiotics used in vaccine manufacture). If you look up some vaccine ingredients on the internet you may read that they could be harmful, but most of them are present in vaccines in amounts that are completely normal for our bodies. Even common salt (sodium chloride), which is essential for the normal functioning of the body, is harmful in large quantities.