Infant vaccinations and risk of childhood acute lymphoblastic leukaemia in the USA.

Previous studies have suggested that infant vaccinations may reduce the risk of subsequent childhood leukaemia. Vaccination histories were compared in 439 children (ages 0-14) diagnosed with acute lymphoblastic leukaemia (ALL) in nine Midwestern and Mid-Atlantic states (USA) between 1 January 1989 and 30 June 1993 and 439 controls selected by random-digit dialing and individually matched to cases on age, race and telephone exchange. Among matched pairs, similar proportions of cases and controls had received at least one dose of oral poliovirus (98%), diphtheria-tetanus-pertussis (97%), and measles-mumps-rubella (90%) vaccines. Only 47% of cases and 53% of controls had received any Haemophilus influenzae type b (Hib) vaccine (relative risk (RR) = 0.73; 95% confidence interval (CI) 0.50-1.06). Although similar proportions of cases (12%) and controls (11%) received the polysaccharide Hib vaccine (RR = 1.13; 95% CI 0.64-1.98), more controls (41%) than cases (35%) received the conjugate Hib vaccine (RR = 0.57; 95% CI 0.36-0.89). Although we found no relationship between most infant vaccinations and subsequent risk of childhood ALL, our findings suggest that infants receiving the conjugate Hib vaccine may be at reduced risk of subsequent childhood acute lymphoblastic leukemia. Further studies are needed to confirm this association and, if confirmed, to elucidate the underlying mechanism.