title: Risk factors for breakthrough varicella in healthy children.
abstract: OBJECTIVE
  To evaluate the risk factors for breakthrough varicella in a follow up study of a cohort of 181 healthy children immunised when aged 9-24 months with a reformulated Oka strain varicella vaccine (SmithKline Beecham Biologicals/Oka).
  
  
  METHODS
  The children were randomised in a double blind manner into one of four groups to receive one of two production lot vaccine batches, at two different titres (high titre, 10(3.9) and 10(4.0) plaque forming units (pfu); low titre (heat exposed), 10(2.7) and 10(2.8) pfu). The overall seroconversion rate after immunisation was 99%.
  
  
  RESULTS
  One hundred and sixty-eight patients were available for review after a mean (SD) follow up of 35 (9) months after vaccination. Multivariate analysis indicated that risk factors for breakthrough varicella were household contact with varicella (adjusted odds ratio (OR), 19.89; 95% confidence interval (CI), 18.39 to 21.39), vaccination age of < or = 14 months (adjusted OR, 2.30; 95% CI, 1.69 to 2.90), and receiving low titre (10(2.7) pfu) vaccine (adjusted OR, 2.13; 95% CI, 1.54 to 2.73). All children who developed breakthrough varicella, had a modified varicella illness, except for three, all of whom had received low titre vaccine.
  
  
  CONCLUSIONS
  The identification of young immunisation age (< or = 14 months) and low titre vaccine as risk factors for breakthrough varicella have important implications for the implementation of varicella vaccination programmes in healthy children.
authors.affiliation: Department of Paediatrics, National University of Singapore, Singapore.
authors.forename: Y J
authors.initials: YJ
authors.lastname: Lim
authors.forename: F T
authors.initials: FT
authors.lastname: Chew
authors.forename: A Y
authors.initials: AY
authors.lastname: Tan
authors.forename: B W
authors.initials: BW
authors.lastname: Lee
mesh_terms: D000367:Age Factors; D016009:Chi-Square Distribution; D002644:Chickenpox / Q000276:immunology / Q000517:prevention & control* / Q000635:transmission; D019433:Chickenpox Vaccine / Q000008:administration & dosage* / Q000276:immunology; D002675:Child, Preschool; D004311:Double-Blind Method; D005192:Family Health; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007223:Infant; D008297:Male; D012307:Risk Factors; D014611:Vaccination
chemical_list: D019433:Chickenpox Vaccine