We investigated a chickenpox outbreak that started in an Oregon elementary school in October 2001, after public schools began phasing in a varicella vaccination requirement for enrollment.
We sought to determine the rate of varicella vaccination and effectiveness and risk factors for breakthrough disease.
Of 422 students, 218 (52%) had no prior chickenpox. Of these, 211(97%) had been vaccinated before the outbreak.
A chickenpox outbreak occurred in a school in which 97% of students without a prior history of chickenpox were vaccinated.