Avidity of tetanus and Hib antibodies after childhood acute lymphoblastic leukaemia - implications for vaccination strategies

AIM: To investigate the possible relationship between serum levels and avidities of antibodies against tetanus toxoid (TT) and Haemophilus influenzae type b (Hib) in children that were vaccinated after treatment for childhood acute lymphoblastic leukaemia (ALL). METHODS: The study groups were 31 paediatric patients with ALL and 18 healthy controls. All subjects were vaccinated with TT and a protein conjugated Hib vaccine. Antibody levels were analysed at three time points: At diagnosis of ALL, after cessation of treatment before vaccination and three weeks after vaccination. Avidity was measured twice, with a thiocyanate elution assay, at diagnosis of ALL and three weeks after vaccination. RESULTS: There was a correlation between level and avidity of tetanus antibodies after vaccination (r(s) = 0.59, P < 0.001). In the standard-risk and intermediate-risk ALL groups, all patients responded with protective levels of tetanus antibodies with normal avidity. In the high-risk ALL group 7/9 patients had subprotective levels of tetanus antibodies after vaccination and concomitantly the lowest avidity, implying poor protection against tetanus. No patients were found with low levels and low avidity of anti-Hib IgG, and 29/31 patients had full protection after a single dose of conjugated Hib-vaccine. CONCLUSION: The vaccination strategy after childhood ALL must be different for low-risk and high-risk ALL groups, since the high-risk group fail to elicit a recall response to tetanus.