Background
A chickenpox (varicella) outbreak occurred in a preschool in Nanjing, between October 2019 and January 2020. This investigation aimed to identify risk factors of the outbreak and assess varicella vaccine effectiveness (VE).

Methods
A varicella case was defined as a student or staff member in the preschool with acute onset of diffuse (generalized) maculopapulovesicular rash without other apparent cause from October 17, 2019-January 15, 2020. A breakthrough varicella case was defined as a varicella case that has been vaccinated for more than 42 days. We evaluated the VE by retrospective cohort study.

Results
This outbreak lasted two months and occurred 5 generations with a median generation interval of 15 days. A teacher triggered this outbreak. She was sick and remained working till the day she was diagnosed. We retrospectively identified 61 cases (55 clinically diagnosed and 6 laboratory diagnosed). Of all the cases, 59 (96.72%) were students from 9 classrooms in 3 grades. The attack rate (AR) among students (10.87%, 59/543) was higher than staff members (2.94%, 2/68) (P=0.040). There were eight (13.11%, 8/61) breakthrough varicella cases, five of which were vaccinated with one-dose more than 5 years. Three classrooms with higher ARs were selected for VE assessment, including 111 students without varicella history. Of the 111 students, 35 (31.53%) were vaccinated with one-dose. The VE was estimated as 65% (95% confidence interval=24%-84%). Subgroup analysis showed that the VE was 71% for the students vaccinated with one-dose within 3 years and 63% for the students vaccinated with one-dose more than 3 years.

Conclusion
Lacking health surveillance and self-report of staff members, and low vaccination coverage contribute to the transmission. VE decreased with the time after one-dose vaccination. We recommended offering two doses of vaccine and strengthening health surveillance for both students and staff members in school.