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Letters to the Editor

Indian Pediatrics 1998; 35:685-686

Immunogenicity of Haemophilus Influenzae Type b Vaccine in Indian Infants


There are geographic, sometimes ethnic, differences in immune response to certain vaccines, and Hib vaccine is one of them. Therefore it is important to investigate the immunogenicity of Hib and any other vaccines that are newly introduced in our country. As there are several Hib vaccines, when any of them is introduced, it must be specifically evaluated. It is in this back- ground that I appreciated the study of Prof. Phadke and colleagues, on the immunogenicity of HbOC (trade name Hib TITER), published recently(1).

The results of the study showed reasonably good antibody response of infants after 3 doses of HbOC. However, there are a few unanswered issues, the answers to some of which could be teased out from the results of this study itself. For other issues, further studies are essential. One question is about the interval between doses of HbOC that we should recommend in India. If immune response is significantly better when the inter-dose interval is increased to 8 weeks, then we should re-consider whether other vaccines (e.g., DPT) could also be given at such intervals in order to accomodate HbOC within the routine infant immunization schedule. This would avoid giving DPT and HbOC out of step with each other. Among the 103 infants who completed the study, some had received the doses at 4 or 8 weeks intervals(1). If the infants are divided into those who received any 2 doses at 8 weeks interval and those who received all 3 doses at 4 weeks intervals, the geometric mean antibody titres (GMT) in them could be compared. The remaining infants would have received their doses at intermediate intervals. If these 3 groups show high, low and intermediate GMT, then we will know that 4 weeks interval is not the best.

Another issue is the prevalence of, and the age distribution of, protective levels of antibody before immunization. In 33 infants below 16 weeks
, ≥ 10.15 mcg/ml antibody titres were noted. What were their exact age and exact titers? Such information will help us in formulating the best age at which the first dose is to be recommended.

If the antibody response of these 33 infants is examined independent of the remaining 65 infants, then. we could see if the antibody in the former was of maternal origin (in which case we expect post-immunization antibody levels equal to or less than those achieved by infants without pre-vaccination antibody) or of their own, consequent upon colonisation by Hib (in which case we expect high antibody titers due to booster effect).

I shall use this opportunity to make three further comments. Since only 75% of vaccinated infants achieved long term protection (antibody titer of
1.0 mcg/ml), a booster dose of HbOC at 12 months will be advisable for ensuring protection upto 5 years of age, which is the period of risk for invasive disease with Hib. Second, this study is unsuitable for assessing reactogenicity of Hib vaccine, since. every infant was given DPT also simultaneously. There- fore mild fever in 56% and irritability in 18.4% cannot be attributed to Hib vaccine. Finally, it was stated that either vaccine was given in the thigh or buttock. In the West from where most data on Hib vaccines have emerged, injections are usually not given in the gluteus. Although there is no evidence that Hib or DPT vaccine induces less antibody response when given in the gluteus than in the quadriceps, such a possibility cannot be completely ignored. In order to make comparisons between studies more reliable, I would suggest that
injections should not be given in the gluteus.
 

T. Jacob John,
Thekkekara,
2/91 E 2, Kamalakshipuram,
Vellore,
Tamil Nadu - 632 002,
 India.


 

References

1. Phadke MA, Kulkarni MV, Sovani VB, Lokeshwar MR, Venkataraman R. Immunogenicity study of Haemophilus influenzae type b conjugate vaccine in Indian infants. Indian Pediatr 1997; 34: 779-783.
 

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