The side effects of the chickenpox vaccine are mild. About 20 to 30 percent of recipients develop a sore arm from the vaccine. About 10 to 15 percent of recipients develop a fever, but interestingly, the same number of recipients who received a saline injection also developed a fever. This suggests that the vaccine is not the cause of this fever. As always, whenever children develop a fever, a few of them might develop a febrile seizure, a seizure caused by the fever.
In addition, it is possible to actually develop a mild case of chickenpox from the vaccine with an associated rash. Remember, this is milder than the wildvirus case of chickenpox. Finally, there is a rare risk of pneumonia and other severe complications, but again, it is not always clear that the vaccine caused the complication.