title: Active surveillance of adverse events following childhood immunization in Singapore.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Paediatric Medicine, KK Women's and Children's Hospital, 100 Bukit Timah Road, Singapore 229899, Singapore; Duke-National University of Singapore Graduate Medical School, 8 College Road, Singapore 169857, Singapore; Yong Loo Lin School of Medicine, National University of Singapore, 1E Kent Ridge Road, National University Health System Building (NUH), Singapore 119228, Singapore. Electronic address: Thoon.Koh.Cheng@kkh.com.sg.
authors.forename: Koh Cheng
authors.initials: KC
authors.lastname: Thoon
authors.affiliation: Vigilance Branch, Health Products Regulation Group, Health Sciences Authority, 11 Biopolis Way #11-01 Helios Singapore 138667, Singapore. Electronic address: Sally_SOH@hsa.gov.sg.
authors.forename: Sally Bee Leng
authors.initials: SB
authors.lastname: Soh
authors.affiliation: Department of Paediatric Medicine, KK Women's and Children's Hospital, 100 Bukit Timah Road, Singapore 229899, Singapore. Electronic address: woei_kang@yahoo.com.sg.
authors.forename: Woei Kang
authors.initials: WK
authors.lastname: Liew
authors.affiliation: Department of Paediatric Medicine, KK Women's and Children's Hospital, 100 Bukit Timah Road, Singapore 229899, Singapore. Electronic address: arunan.gunachandran@gmail.com.
authors.forename: Arunan
authors.initials: A
authors.lastname: Gunachandran
authors.affiliation: Department of Paediatric Medicine, KK Women's and Children's Hospital, 100 Bukit Timah Road, Singapore 229899, Singapore; Duke-National University of Singapore Graduate Medical School, 8 College Road, Singapore 169857, Singapore; Yong Loo Lin School of Medicine, National University of Singapore, 1E Kent Ridge Road, National University Health System Building (NUH), Singapore 119228, Singapore. Electronic address: Natalie.Tan.WH@kkh.com.sg.
authors.forename: Natalie Woon Hui
authors.initials: NW
authors.lastname: Tan
authors.affiliation: Department of Paediatric Medicine, KK Women's and Children's Hospital, 100 Bukit Timah Road, Singapore 229899, Singapore; Duke-National University of Singapore Graduate Medical School, 8 College Road, Singapore 169857, Singapore; Yong Loo Lin School of Medicine, National University of Singapore, 1E Kent Ridge Road, National University Health System Building (NUH), Singapore 119228, Singapore. Electronic address: Chong.Chia.Yin@kkh.com.sg.
authors.forename: Chia Yin
authors.initials: CY
authors.lastname: Chong
authors.affiliation: Department of Clinical Epidemiology, Communicable Disease Centre, Institute of Infectious Disease and Epidemiology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore. Electronic address: cheefu.yung@gmail.com.
authors.forename: Chee Fu
authors.initials: CF
authors.lastname: Yung
mesh_terms: D016907:Adverse Drug Reaction Reporting Systems; D001500:BCG Vaccine / Q000009:adverse effects; D015984:Causality; D005260:Female; D006801:Humans; D007223:Infant; D008199:Lymphadenitis / Q000139:chemically induced; D008297:Male; D011159:Population Surveillance / Q000379:methods*; D012846:Singapore; D014611:Vaccination / Q000009:adverse effects*
chemical_list: D001500:BCG Vaccine
keywords: Adverse events following immunization; Children; Singapore; Vaccines