Nowhere near the danger from the disease. About a hundred people used to die from the varicella virus each year in the US. The vaccine for varicella was introduced in 1995; in a dozen years the number of deaths had been reduced to six. The number of deaths in the US from the vaccine has been zero.
Was there anything different about those 100 that you could predict who the chicken pox would harm and who it wouldn’t? Why not just vaccinate those people at risk?