What is the likelihood that a child under one year old will catch chicken pox in the u.S.?

Less than 5%.... ..According to the cdc, because most american children are now vaccinated against the disease. Most cases these days are in children age 4 or younger, either because they haven't been vaccinated (the first shot is usually given at age 1), or because they are in the small group (3%) who are vaccinated and get chickenpox anyway.