Originally, ACIP only recommended one dose of varicella vaccine for children. Why did ACIP later revise its recommendations to add a second dose of varicella vaccine for all children?

In the ten years following vaccine licensure in 1995, there was a significant decline in varicella disease, as well as varicella-related hospitalizations and deaths. Although a 1-dose regimen was estimated to be 80% to 85% effective, breakthrough disease was still occurring in highly vaccinated populations. A 2-dose regimen was adopted in 2006 to further reduce the risk of disease among vaccinated people whose numbers would accumulate over time, which could lead to varicella disease later in life when it can be more severe.