Studies have shown that when adults have a clear memory, either personal or from a parent, of having had chickenpox, they are over 99 percent likely to be immune. Interestingly, though, if an adult does not have any memory of having had chickenpox, he or she still has a 70 percent chance of being immune. This suggests that many people have such mild cases of chickenpox that they go unnoticed.
With this information in mind, for those who do not have any recollection of having had chickenpox in the past, there are two options. They can have their blood drawn and only receive the vaccination if they are not immune. Or they can simply receive the vaccinations without any blood test. Adults should receive two doses at least four weeks apart.