A multicentre trial of live attenuated varicella vaccine in children with leukaemia in remission.

Two hundred forty children with acute leukaemia in remission for at least 1 year were immunized with live attenuated varicella vaccine. All were susceptible to varicella before immunization. There was a seroconversion to varicella-zoster virus in approximately 85% after 1 dose, and in 97% after 2 doses. The major side effect was mild to moderate rash, seen mainly in children with maintenance chemotherapy suspended for 1 week before and 1 week after vaccination. Vaccinees with rash were at some risk (10%) to transmit vaccine virus to varicella susceptibles with whom they had close contact. Twenty-nine vaccinees were subsequently exposed to varicella in their households. The attack rate of clinical varicella in these vaccinees was 21%, which is significantly lower than the 80%-90% attack rate occurring in varicella susceptibles after household exposure. All these breakthrough cases of varicella were mild, even in leukaemics receiving chemotherapy. Varicella vaccine was approximately 80% effective in preventing clinical varicella in children with leukaemia and completely effective in preventing severe varicella in this high-risk group.