Among those without a history of varicella in grade 1, there were 4 cases out of the 15 unvaccinated students (ARU: 26.7%) and 9 out of the 123 vaccinated students (ARV: 7.3%). Therefore, vaccine effectiveness was 72.6% for any varicella illness in grade 1.
It follows that vaccine effectiveness was 69.3% in grade 3 and 100.0% in grade 4.
Overall vaccine effectiveness against moderate or severe varicella was 85.5%.
Of the 16 cases in the outbreak, 10 (62.5%) cases occurred in vaccinated students.
There were 5 (83.3%) cases with at least 50 lesions in unvaccinated students with varicella disease, compared with 2 (20.0%) cases with at least 50 lesions among vaccinated students (RR: 4.17; 95% CI: 1.15, 15.14).
The median duration of the illness in vaccinated students was 6.0 days (range: 2~10days), compared with 8.5 days (range: 6~15days) among unvaccinated students (p = 0.370). No child required hospitalization.