The chicken pox virus (that also causes shingles in adults) doesn't 'clear' after we've had chicken pox as kids. It lives in our dorsal spinal column. When our immune surveillance system is on line - our immune system keeps it in a dormant state. However, at times of stress or other illness - when our immune system is 'down' -- it can overcome the surveillance and 'pop out'. Think cold sores - exacerbations are common with stress. The 'side effects' of vaccination is evidence that your immune system has recognized the vaccine & is building antibodies. The fever, chills, lethargy are all common. Even as kids we got fevers after having a vaccination. None of that is breaking news. At the time my charge nurse had her difficulties - the vaccine wasn't available. (Yes, I've been a nurse THAT long!- LOL)