When I started in practice, I had several kids a year who had severe complications of chickenpox. Now, I rarely even see a case of chickenpox. I have had a few kids who had mild cases after immunization. I am not sure of a percentage, but it can't be higher than 1-5% at most. The incidence of shingles also seems to be much less than it was 15 years ago when I started. Another interesting observation is that I have never seen a single case of measles since I started medical school. I have seen rashes that looked like they could have been measles, but turned out not to be. By the time I retire, perhaps, there will be a new generation of doctors who have never seen chickenpox.