Letters to the Editor

Indian Pediatrics 1998; 35:686-688

Reply

We thank Dr. Jacob John for his comments and suggestions which are well taken. We have tried to re-analyse our data based on his suggestions.

On analysis of the infants based on the preimmunization titer, it was found that the infants having pre-immunization titer ≥ 0.15 mcg/ml (n = 33) had lesser degree of rise in the titer following immunization with Hib as compared with their other counterpart, i.e., those with preimmunization titer < 0.15 mcg/ml (n = 70). However, this difference was not statistically significant (Table 1).

When tabulated for the age at first dose in weeks against the basal anti PRP titers, there was no trend towards an increasing titer with increasing age at first dose (Table II). Therefore the higher titers are more likely to have come from the mothers. The percentage of children with a basal level ≥ 0.15 mcg/ml in this study is consistent with published international data(1).

As suggested we did regroup the infants according ,to the interval between doses. Only one child had been vaccinated at exactly 4 weeks interval between three doses and no infant had an interval of 8 weeks between doses. Due to this fact, such analysis was not feasible. Although we agree with the thought of doing such an analysis, we may need to plan this out in a larger study.

We would also like to point out that apart from dosage interval, there are many other factors that influence Hib immune responses. For example, for Hib oligosaccharide-CRM 197 conjugate vaccine (HbOC) (HibTITER), a direct relationship has been noted between the magnitude of the immune response and increasing age, up to approximately 13 months of age, following a single 10 mcg dose of HbOC(I,2). There is also a direct relationship between increasing age and percentage of children with titers greater than 1 mcg/ml following one dose' of HbOC(2). Avidity (i.e., binding power), which is a measure of the func- tional capacity of antibody response, is also an important factor. HbOC stimulates anti- bodies with high avidity, suggesting that lower levels than l,mcg/ml are potent functionally(3). In addition, for primary series Hib vaccination, protective antibody levels have been noted with inter-dose intervals as one month between doses(4).
 

V.B. Sovani,
Medical Director,
Wyeth Lederle Limited, Mumbai,
India.