The varicella vaccine is a shot that can prevent chickenpox. It is called varicella because the varicella virus causes chickenpox. Up to 90% of people who receive the vaccine will not get chickenpox.People who get chickenpox after having the vaccine have a milder form of the disease.t is most common in children. Most cases occur in people who are younger than 15 years of age. Chickenpox is usually a mild illness, but it can cause problems such as brain swelling, pneumonia and skin infections. Some children get sicker than others. Chickenpox may be a very serious illness in infants and adults.