The first definition of the word safe is "harmless." This definition would imply that any negative consequence of a vaccine would make the vaccine unsafe. Using this definition, no vaccine is 100 percent safe. Almost all vaccines can cause pain, redness or tenderness at the site of injection. And some vaccines cause more severe side effects. For example, the original pertussis (whooping cough) vaccine could cause persistent, inconsolable crying, high fever or seizures associated with fever. Although none of these severe symptoms resulted in permanent damage, they could be quite frightening to parents.
But, in truth, few things meet the definition of "harmless." Even everyday activities contain hidden dangers. For example, every year in the United States, 350 people are killed in bath- or shower-related accidents, 200 people are killed when food lodges in their windpipe, and 100 people are struck and killed by lightning. However, few of us consider eating solid food, taking a bath, or walking outside on a rainy day as unsafe activities. We just figure that the benefits of the activity clearly outweigh the risks.
The second definition of the word safe is "having been preserved from a real danger." Using this definition, the danger (the disease) must be significantly greater than the means of protecting against the danger (the vaccine). Or, said another way, a vaccine's benefits must clearly and definitively outweigh its risks.
To better understand the definition of the word safe when applied to vaccines, let's examine four different vaccines and the diseases they prevent.
Hear Dr. Offit explain the risks and benefits of vaccines by watching this short video, part of the Talking About Vaccines with Dr. Paul Offit video series.
Reviewed by Paul A. Offit, MD, Lori Handy, MD, MSCE on August 11, 2017