No matter how much is known about a vaccine before it is licensed for routine use, after it has been used in millions of people it is possible to discover side effects that are too subtle or too rare to show up in pre-license testing. The November 2000 issue of Infectious Diseases in Children reviewed the post-licensure report card of the varicella (chicken pox) vaccine. Whenever an unexpected event follows an immunization, we physicians are required to report it to the Vaccine Adverse Events Reporting System (VAERS), even if we think the event was not caused by the vaccine.
The chicken pox vaccine appears to be quite safe. Even if all of the bad things that happened after the shot were caused by the shot (unlikely), complications were still much less than what would be expected if no immunization had been given. No reaction occurred more often than occurs during chicken pox itself. I suspect that the individuals who do react to the weakened formed of the virus in the vaccine may be the same ones who would have an even worse reaction to the stronger virus in chicken pox itself.
This report card is very encouraging, but not definitive. Not all adverse events are recognized by doctors and not all of those that are recognized are reported. The complication rates for both the disease and the vaccine are probably under-estimated. On the other hand, many children receive several immunizations at the same time, and thus many reactions listed as possibly coming from the chicken pox vaccine have no relationship at all.
What safety news we have about the vaccine at this point is good. With any health decision, it is wise to evaluate both the possible risks and the potential benefits. While serious complications from chicken pox are uncommon, chicken pox remains the number one cause of vaccine-preventable death in the United States.