Abstract

AIM 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).

DESIGN 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 ⩽ 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.

CONCLUSION The identification of young immunisation age (⩽ 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.