A summary of case-control studies conducted from 1997 to 2003 showed that a single dose of varicella (chickenpox) vaccine was 97% effective in the first year after vaccination and 86% effective in the second year. From the second to eighth year after vaccination, the vaccine's effectiveness remained stable at 81% to 86%. Most vaccinated children who developed chickenpox during the eight years after vaccination had a mild case of the disease.
Why do most vaccinated individuals, if they are infected, show only a mild case of the disease? (2 marks)