The two-dose series is extremely effective, with over 99 percent of recipients fully protected against the virus. However, part of the effectiveness of the vaccine in the past was due to regular reexposure to the wild virus in the community. In essence, being exposed to another person in the community with chickenpox was like getting a booster dose of the vaccine.
With the number of cases of community-acquired chickenpox decreasing, there is some concern that additional booster doses will be necessary in the future. This has already occurred with the recommendation changing from one dose at twelve to fifteen months to include a booster dose at kindergarten. Because widespread vaccination did not start until the late 1990s, it is unclear whether the chickenpox will just require two doses, like the MMR vaccine, or whether boosters will be needed every ten years, like the tetanus vaccine. Only time will tell.
Another question about the vaccine is whether or not it will protect against shingles. This question will take decades to answer. In the past, most cases of chickenpox occurred in childhood, and most cases of shingles occurred in adults over age fifty. Thus, it will take years to decide if the vaccine significantly decreases the risk of shingles.