Why did I get chickenpox even though I've been vaccinated?

Previous vaccination with the varicella vaccine does not always guarantee that a person will not develop chickenpox symptoms. However, if they do, these symptoms are usually much milder than in an unvaccinated person. About 15% – 20% of people who have received one dose of varicella vaccine will still get chickenpox if they are exposed to the virus. Children who have received two doses of the vaccine have a less than 7% chance of getting chickenpox.

The effectiveness of vaccination depends upon how many doses of the vaccine have been given, and how strong the person's immune system is. According to the Centers for Disease Control and Prevention (CDC), one dose of varicella vaccine is 82% effective at preventing any form of chickenpox, and 100% effective at preventing the severe form of the disease. Two doses of varicella vaccine increase the effectiveness against any form of varicella to 98%. Effectiveness rates may be lower in people with other health conditions such as HIV or who are immunocompromised (for example, taking medications that suppress the immune system).

Chickenpox that occurs in a person who has been previously vaccinated against the disease is called “breakthrough varicella”, and it is usually mild. People with breakthrough varicella typically don’t have a fever, or only have a low-grade fever, and develop fewer than 50 skin lesions. The rash tends to be milder and lacks the vesicles typically seen in unvaccinated people who develop the disease. People with breakthrough varicella also tend to recover quicker than unvaccinated people.

Vaccination is generally long-lasting although experts aren't sure exactly how long it lasts. Some studies have shown antibodies against varicella are still present 10 to 20 years after vaccination.