In general, live vaccines cause side effects similar to a mild case of the corresponding natural infection. Thus, measles and varicella vaccines can cause a mild rash or fever in a minority of vaccine recipients. But these reactions are either temporary or so rare as to be overshadowed by the consequences of the disease being prevented. If FDA determined that the benefits of a vaccine did not outweigh its side-effect profile, FDA would revoke the manufacturer s license to distribute the vaccine. The sensation when a needle pierces the skin is probably best described by the word sting , rather than pain .
Nerves are in the surface of the skin.
Once a needle passes through the outer layer of the skin, there is no more sensation. After the needle enters the body, one can let a syringe dangle unsupported without causing any pain. After any fluid is pushed through the needle into the tissue or muscle, the body has to adjust to the presence of that fluid. As a result, the injection site may become sore or tender after a few hours. Redness and hardness close to the needle-entry site are not uncommon.