This study compared the immune responses of healthy children to one or two doses of Oka/Merck varicella vaccine. Both cell-mediated and humoral immune responses were significantly stronger 1 year later among children given two doses 3 months apart than among those given a single dose. Long-term follow-up studies are needed to determine whether two doses of vaccine indeed provide more complete and more durable protection than a single dose against breakthrough varicella in healthy children.
The purpose of the study was to establish the minimum immunizing dose of stored Oka/Merck varicella vaccine and mimic effects of prolonged storage of vaccine at freezer temperatures. Ninety children were randomized to one of three groups: group A (1770 pfu/dose), group B (400-500 pfu/dose), or group C (80-160 pfu/dose). Seroconversion rates for groups A-C were 97%, 96%, and 92%, respectively. Group A and B vaccinees had higher antibody titers at 1 year than did group C subjects (P = .01). The difference between groups B and C approached significance (P = .058). At 1 year after immunization, 73%, 83%, and 80% in groups A-C, respectively, demonstrated cell-mediated responses. Since freshly reconstituted vaccine contains 1770 pfu/dose, vaccine failure from loss of potency is unlikely if vaccine is stored properly and administered promptly after reconstitution.
Oka/Merck varicella vaccine has been studied in this institution since 1981. Persistence of antibody for 6 to 8 years has been demonstrated; however, cases of chickenpox have been seen in immunized children. The severity of chickenpox in healthy children who have received Oka/Merck varicella vaccine since 1981 is described. All vaccinees who developed chickenpox-like rashes more than 6 weeks postimmunization were exammined. Of 2163 vaccinees, 164 were examined, of whom 114 had rashes consistent with chickenpox. When sera were available (46%), antibody studies uniformly confirmed varicella-zoster virus infection. Chickenpox occurred 2 to 96 months (median of 44 months) postimmunization. The range for the number of skin lesions was 1 to 285 (median 18) in seroconverters. Symptoms included itching in 39%, fever in 9%, headaches in 7%, lymphadenopathy in 3%, and malaise in 2%; 54% were asymptomatic, except for the rash. The median time to total healing was 5 days. The median time lost from school was 2 days. Thirteen of the children in whom infections developed had failed to seroconvert after immunization. Their infections were similar in severity to those of children who had seroconverted originally. When varicella was introduced into families as a result of chickenpox in an immunized family member (index case), the rate of secondary chickenpox among immunized siblings was 12.2%. Eleven such secondary cases were similar in severity to the 9 index cases. It is concluded that chickenpox is generally mild in previously immunized children.
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