SINGLE DOSE OF 0.5 ML IS GIVEN SUBCUTANEOUSLY TO CHILDREN AGED 12 MONTHS TO 13 YEARS. PERSONS OVER THE AGE OF 13 YEARS SHOULD RECEIVE TWO DOSES AT AN INTERVAL OF 4 TO 8 WEEKS.
RESULTS OF STUDIES OF VARICELLA-ZOSTER VACCINES IN HEALTHY AND LEUKAEMIC CHILDREN HAVE BEEN LARGELY FAVOURABLE. PROTECTIVE EFFICACY IN HEALTHY CHILDREN APPEARS TO BE OVER 90%. IN HEALTHY ADOLESCENTS AND ADULTS, ADDING A SECOND DOSE 4 OR 8 WEEKS AFTER THE FIRST INCREASED SEROCONVERSION RATES FROM ABOUT 70 TO 80% TO 97% OR BETTER.1 A PROTECTIVE EFFICACY OF ABOUT 85% HAS BEEN REPORTED IN LEUKAEMIC CHILDREN GIVEN ONE DOSE OF VARICELLA-ZOSTER VACCINE.2,3 A TWO-DOSE REGIMEN (DOSES SEPARATED BY 3 MONTHS) HAS BEEN TRIED. ALTHOUGH A SECOND DOSE IS PROBABLY OF BENEFIT TO THOSE WHO DO NOT SEROCONVERT AFTER ONE DOSE, ITS ADDITIONAL BENEFIT TO THOSE WHO DO SEROCONVERT IS DOUBTFUL.3 INTERRUPTION OF CHEMOTHERAPY FOR VACCINATION DOES NOT APPEAR NECESSARY IN TERMS OF IMMUNOGENICITY OF THE VACCINE.2,4 A HIGHER RATE OF VACCINE-ASSOCIATED RASHES HAS, HOWEVER, OCCURRED IN LEUKAEMIC CHILDREN WHEN CHEMOTHERAPY WAS SUSPENDED FOR 2 WEEKS COMPARED WITH THOSE WHOSE CHEMOTHERAPY HAD PREVIOUSLY BEEN TERMINATED; THIS ONLY OCCURRED WITH CERTAIN LOTS OF VACCINE.2
THE DURATION OF IMMUNITY AFTER VARICELLA-ZOSTER VACCINATION IS ALSO UNDER DEBATE. STUDIES SHOW THAT THE IMMUNITY INDUCED BY NATURAL INFECTION WITH WILD TYPE VIRUS IS SUPERIOR TO THAT INDUCED BY THE VACCINE. IN ONE STUDY,3 ABOUT ONE-QUARTER OF ALL VACCINEES (BOTH LEUKAEMIC CHILDREN AND HEALTHY ADULTS) WERE SERONEGATIVE 1 YEAR AFTER A SECOND DOSE OF VACCINE, BUT NONE WERE SERONEGATIVE AFTER UP TO 6 YEARS AFTER BREAKTHROUGH VARICELLA INFECTION. HOWEVER, IMMUNITY TO VARICELLA-ZOSTER IS COMPLEX, DEPENDING NOT ONLY ON CIRCULATING ANTIBODY BUT ALSO ON CELLULAR IMMUNITY AND SECRETORY ANTIBODY. THUS, ALTHOUGH A PERSON MAY BECOME SERONEGATIVE AFTER VACCINATION, PROTECTION FROM VARICELLA MAY REMAIN, ALBEIT PARTIAL.2 BOTH HUMORAL AND CELL MEDIATED IMMUNITY HAVE BEEN SHOWN TO PERSIST FOR UP TO 20 YEARS AFTER VACCINATION.5 LEUKAEMIC CHILDREN OBSERVED UP TO 6 YEARS AFTER IMMUNISATION HAVE CONTINUED TO BE WELL PROTECTED2 AND VARICELLA IN PREVIOUSLY-VACCINATED PERSONS IS USUALLY MILD.2,3