Field investigation of a mumps outbreak and vaccination effect evaluation in a school in Anhui, 2018–2019

ObjectiveTo understand the cause of a mumps outbreak and evaluate the vaccination effect (VE) of mumps-containing vaccine (MuCV) in a school in Anhui province during 2018–2019, and provide scientific basis for the improvement of immunization programs against mumps.

MethodsDescriptive epidemiology method was used to analyze the cause and characteristics of this mumps outbreak. A retrospective cohort study was conducted to evaluate the vaccination effect of MuCV.

ResultsA total of 78 mumps cases were identified during the outbreak, and the overall attack rate was 11.24% (78/694). The initial case had a history of a two-dose MuCV vaccination before the outbreak, and no standard isolation was performed for 42.31% of the cases. 91.03% cases were students in No.1 teaching building. The coverage of MuCV in students from No.1 and 2 teaching buildings were 90.25% and 68.38%, respectively. The incidence risk of student from No.1 teaching building was 3.71 times higher than that of students from No.2 teaching building (RR＝3.71, 95% CI: 1.74–7.88). The protection rate of one-dose, two-dose and three-dose of MuCV were 30.94%, 32.69% and 100.00%, respectively. The longer the interval between onset and vaccination was, the higher the attack rate was. The difference was significant (χ_trend²＝24.472, P＜0.001). The protection effect of MuCV decreased as time pass by, and the protection rate against mumps within 5 years after vaccination was 86.38% (95% CI: 40.29%–96.89%), but the difference in protective effect between vaccination group (＞5 years) and non-vaccination group was not significant (RR＝0.76, 95% CI: 0.41–1.41, P＞0.05).

ConclusionThe cases with a two-dose MuCV vaccination might be the infection source in this outbreak too. Non-standard management of the mumps cases in this school and waning immunity conferred by MuCV were likely contributors to this outbreak. Patients with two-dose of MuCV vaccination are still contagious. MuCV only provide effective protection against mumps within 5 years after vaccination. If the vaccination time is longer than 5 years, the protection effects of both one-dose and two-dose of MuCV are poor, however, three-dose of MuCV has better protection and immune persistence.