OBJECTIVE:To assess the reactogenicity and safety of a thermostable, high titre, varicella vaccine in healthy infants and children. DESIGN: Open study of 505 children monitored for 42 days after vaccination. SETTING: Three urban Canadian centres (Halifax, Ottawa and Vancouver). PARTICIPANTS: 505 healthy children one to 12 years of age were enrolled and 504 completed the study. All were susceptible to varicella by history. INTERVENTIONS: All participants received one dose of live attenuated varicella vaccine (1×10 4.5 plaque forming units/dose) subcutaneously. MAIN OUTCOME MEASURES: The children were monitored from the day of vaccine administration (day 0) until day 42. All local and general symptoms and signs were recorded on diary cards by the patients' parents, who were encouraged to fill in the cards on days 2 to 3 and 18 to 24 via telephone calls from study personnel. RESULTS: Most of the symptoms noted after vaccine administration were mild and transient, and all resolved within the respective follow-up periods. Injection site symptoms included pain (17.5%, 13.9% and 30.4% in centres 1, 2 and 3 respectively), redness (21.1%, 32.1% and 48.8%) and swelling (7%, 10.3% and 29.2%). The general symptoms reported were fever 37.5°C or higher (3.5%, 4.8% and 3.0%) and varicella-like rashes (6.4%, 2.4% and 0%). Two subjects had severe symptoms (one with cervical lymphadenopathy, and one with a fever higher than 39°C) probably related to vaccine administration. No serious adverse events were reported during the entire study. CONCLUSION: The vaccine was well tolerated. I nfection by varicella zoster virus (VZV) usually results in benign disease in children. However, complications such as pneumonia, encephalitis and bacterial superinfection of the skin lesions (1,2) occur in some patients, mainly in adolescents and adults, and in immunocompromised children (1,2). In addition, children born to nonimmune mothers who contract varicella during pregnancy can develop congenital varicella syndrome (1,3), with limb hypoplasia and central nervous system damage.A live varicella vaccine was developed in Japan in 1974 (4) using the OKA strain of the virus. The original wild type virus was isolated from a boy with natural varicella, and then attenuated by passages through human and guinea pig embryonic cells, and two human diploid cell lines, WI-38 and MRC-5 (4,5). The vaccine strain obtained after this treatment has different thermosensitivity and host range spectrum than the wild type virus (6). Additionally, it can be easily differentiated from wild type strains currently circulating in North America by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) testing (6,7).All OKA varicella vaccine production lots are derived directly from a working seed lot (8), following the World Health Organization guidelines (9). Many clinical trials have demonstrated that this strain is safe and immunogenic (2,5,10,11). The vaccine produced by SmithKline Beecham Biologicals (Rixensart, Belgium), VARILRIX, was firs...