Effectiveness of the Oka/GSK attenuated varicella vaccine for the prevention of chickenpox in clinical practice in Israel

Background: The varicella Oka/Merck vaccine has been shown to be very effective in clinical practice; however, several recent studies reported reduced effectiveness. The varicella Oka/GSK vaccine (Varilrix; GlaxoSmithKline Biologicals), which has been subjected to fewer effectiveness studies, was licensed in Israel for voluntary use in 2000. This study was planned to estimate the effectiveness of the varicella Oka/GSK vaccine among children 1-5 years of age. Methods: Recent reports of varicella were retrieved from the Tel Aviv Health District. Two age-matched control subjects who attended the same day-care center and/or resided in the same neighborhood and had not contracted varicella in the past were recruited for each varicella case. The parents of case and control subjects were interviewed regarding the history of varicella, vaccination and the severity of illness. Results: A total of 151 case subjects (mean age, 2.9 +/- 1.1 years) and 298 control subjects (mean age, 3.0 +/- 1.1 years) were recruited. Vaccination rates for case and control subjects were 6.6% and 38.3%, respectively (P < 0.001). Disease among vaccinated children was significantly milder, as measured by several indices, including parental perception of the illness, presence of fever, time until the lesions dried and days of day care missed. Vaccine effectiveness against varicella of any severity with these figures was 88% (95% confidence interval, 77-94%), and effectiveness against moderate/severe illness was 100%. Conclusions: Our data demonstrate that the varicella Oka/GSK vaccine used in clinical practice is highly effective in prevention of varicella (especially moderate/severe disease) among children, including those attending day-care centers.