Chickenpox, or varicella, was once a very common childhood illness but it is preventable. The main factor that would prevent children from contracting chickenpox is vaccination. The varicella vaccine is administered in two doses at different times. The recommended schedule is to administer the first dose when the child is twelve to fifteen months old, and the second dose when they are four to six years old. Older children and adults also need two separate vaccinations. There are some possible side effects that might contribute to spreading the disease. If the child who has received the vaccine develops a rash, it might enable the vaccine virus to spread to an unprotected person. For that reason, anyone who does develop a rash after vaccination should be separated from highly susceptible people such as infants and people with weakened immune systems.
Another reason that some children would not be affected is that they already had the disease, which would have given them immunity. Some people who have had chickenpox were not aware that they had the disease at the time. In such cases, they may take a blood test for varicella-related immunoglobulin G (IgG) antibodies, which will determine if they once had the disease or had been vaccinated.