Active surveillance of adverse events following childhood immunization in Singapore

In Singapore, reporting of adverse events following immunization (AEFI) was historically passive. In 2009, Health Sciences Authority collaborated with KK Women's and Children's Hospital to perform active surveillance for AEFI. We report the methodology and initial findings of this surveillance following childhood vaccines.

From April 2010 to March 2012, we screened all paediatric admissions for possible relationships to vaccination, excluding elective admissions, and performed causality assessment for each case using standardized definitions for certain, probable, possible and unlikely. Baseline demographics, data on implicated vaccines and clinical details including severity and outcomes were collected. Total hospital admissions were used to calculate rates of AEFI.

We screened 45,571 (80%) of 56,526 admissions, and evaluated 1988 (4.4%) children. Median age at presentation was 3.1 months, while median interval from vaccination to symptom onset was 6 days. There were 311 (15.6%) children with AEFI that were considered possibly, probably or certainly associated with vaccines. However, 98.8% recovered without any long-term sequelae. The hospital-based active surveillance of AEFI enabled the detection of a 5-fold increase (95% CI 1.2-33.1) in BCG-associated regional lymphadenitis in April 2010, which triggered follow-up safety analysis to guide public health advice.

Hospital-based active surveillance can enhance signal detection and follow-up investigations of AEFI. Subsequently, public health bodies are better equipped to maintain public confidence in vaccination programmes and physicians are able to provide relevant advice to parents. It also allows for a better understanding of risk-benefit ratios of specific vaccines and aids the generation of public health vaccination policy.