What it prevents: Varicella, or chicken pox, is caused by the highly-contagious varicella-zoster virus. Symptoms begin with a red rash that blisters and crusts over, putting a child at risk for infection. It is also very itchy and can cause scarring when pox are scratched. In severe cases it can cause a serious infection, pneumonia, and even death. The virus can reemerge later in life and cause the related condition known as shingles.

When it’s given: The varicella vaccine is given in two doses. The first is given between 12 and 15 months of age and the second can be given between 4 and 6 years of age or earlier but should be given at least three months after the first dosage. Note: Varicella can be given at the same time as the MMR Vaccine (and is available as a combo shot, the MMRV). However, if it is given after the MMR, it must be more than 30 days afterward to maintain maximum effectiveness.

What you may have heard: Been invited to a “chicken pox party?” Some people would rather expose their child to the chicken pox virus naturally rather than by a vaccine, so they intentionally court the virus. Before the advent of the vaccine, this was a common strategy, since chicken pox is a much more serious disease in adults than it is in children. However, the real disease can cause severe risks for infection and even death. The vaccine contains a weakened form of the virus and is much less likely to cause serious side effects.

Risk of a reaction: Most children will have no side effects; 1 in 5 may have a mild reaction such as redness, soreness, or a few chicken-pox-like bumps at the injection site. 1 in 10 may have a fever, and 1 in 25 may have a rash on other parts of the body for up to a month after vaccination. Side effects are a little more common with the combination vaccine: 1 in 5 may have a fever and 1 in 20 may have a rash. Children are also more likely (though it’s still rare) to have a febrile seizure with the combination shot.