A mass Japanese encephalitis (JE) immunization campaign for children aged 9 months through 12 years was conducted in 2013 in Battambang province, western Cambodia. Vaccinators working at almost 2,000 immunization posts in approximately 800 villages provided vaccinations to almost 310,000 children using one dose of Chengdu Institute of Biological Products' live, attenuated SA14-14-2 JE vaccine (CD-JEV), achieving a coverage rate of greater than 90%. Lessons learned, in general for mass vaccination campaigns and specifically for vaccination with CD-JEV, are described. These observations will be of benefit for public health officials and to help inform planning for future campaigns for JE or other vaccine-preventable diseases in Cambodia and elsewhere.

INTRODUCTION: Japanese encephalitis (JE) virus is the most common cause of vaccine-preventable encephalitis in Asia. The SA14-14-2 JE vaccine manufactured by Chengdu Institute of Biological Products has been shown to be safe and effective in clinical trials and childhood routine immunization programs. However, there are few published reports describing results of surveillance for adverse events following immunization (AEFI) when the vaccine is used in mass campaigns. We describe the results of AEFI surveillance following a 2013 vaccination campaign among almost 310,000 children aged 9 months-12 years in Battambang Province, Cambodia. METHODS: Routine AEFI surveillance was strengthened by staff training and supplemented by active hospital surveillance. An AEFI was defined as any sign, symptom, or disease temporally associated (i.e., within 4 weeks) with receipt of the vaccine, irrespective of whether it was considered related to immunization. Data were collected on standardized forms and causality assessments were conducted for serious AEFI. RESULTS: Passive and active surveillance detected 28 AEFI for an overall incidence of 9.0 AEFI per 100,000 doses administered. The most frequent events were vasovagal episodes (n = 7, 25%) and rash (n = 6, 21%), and most other events were common childhood conditions such as fever and vomiting. Three AEFI were classified as serious, including one hypersensitivity reaction and two meningoencephalitis cases. Of these, the hypersensitivity event was the only serious AEFI classified as being consistent with a causal association to immunization. CONCLUSIONS: Most reported adverse events were conditions that commonly occur after other childhood vaccinations or independently of vaccination, and in the context of careful monitoring for serious AEFI only one serious event consistent with a causal association with immunization was identified. These results support the good safety profile of the SA14-14-2 JE vaccine, and provide reassuring data as the vaccine's use expands.