PERSISTENCE OF ANTIBODY AT 18 MONTHS FOLLOWING VACCINATION OF YOUNG GAMBIAN INFANTS WITH PRP-OMPC HAEMOPHILUS-INFLUENZAE TYPE-B CONJUGATE VACCINE

The rate of decline in anti-PRP antibody levels was measured in two groups of Gambian children who had been given PRP-OMPC at 1 and 3 months or 2 and 4 months of age. In the younger group (n = 70), the geometric mean titre fell from 1.32 mug/ml at 4 months to 0.44 mug/ml at 18 months. In the older group (n = 54), the geometric mean titre fell from 1.18 mug/ml at 5 months to 0.46 mug/ml at 18 months. The proportion of vaccinated children with antibody levels over 1.0 mug/ml fell from 54% 1 month after the second dose of vaccine to 27% at the age of 18 months, while the proportion with levels over 0.15 mug/ml fell from 82% to 60%, with no significant differences observed between the vaccination groups. For those children who did not show evidence of environmental boosting, the half-life of anti-PRP antibody was about 100 days. This did not differ between the groups. These findings suggest that to provide lasting immunity PRP-OMPC should be given with a late booster dose at 12-15 months, as is the current practice in the USA. The need for a late booster dose may limit the value of this vaccine in developing countries where vaccination of children is difficult after the 1st year of life.