.Letter Immune response of infants to inactivated poliovirus vaccine injected intradermally In a recent reappraisal of the live oral poliovirus vaccine (OPV) and the inactivated poliovirus vaccine (IPV), some of the advantages of the latter, particularly for use in developing countries, were highlighted ~. However, one major disadvantage is that at the present time IPV is more expensive than OPV. One way to reduce the cost is to give a fractional dose of IPV intradermally. In an earlier study, we found that there was a striking antibody booster response to intradermal inoculation of a fractional dose of IPV in adults and children who were previously immune2. We now report the results of a preliminary study on immune response to primary immunization with intradermally administered IPV. After obtaining informed consent from parents, 14 infants attending the immunization clinic of our hospital were given two doses (0.1 ml) oflPV (ImovaxPolio, Institut Merieux, Lyon, France) intradermally, as described earlier z, at an 8 week interval starting at 6 weeks of age. Venous blood was collected before the first dose and 4 weeks after the second intradermal dose of the vaccine. Paired sera were tested for the presence and titre of poliovirus neutralizing antibody to all three serotypes by a microneutralization test using 100 TCIDso (acceptable range 32-320) of virus at serum dilutions of 1:4 to 1:2048; the reciprocal of the highest dilution which neutralized the virus was defined as the antibody titre. In seronegative infants, the appearance of antibody after immunization was defined as seroconversion. The presence of antibody in the pre-immunization sera was assumed to be of maternal origin. In such infants the rate of decline of maternal antibody was calculated using a half-life of 4 weeks. Antibody titres four or more times higher than the calculated residual maternal antibody levels in the postimmunization sera were also con-

0264-410X/92/02013.5-O1 © 1992 Butterworth-Heinemann Ltd

Table 1 Neutralizing antibody titres to poliovirus types 1, 2 and 3 prior to, and 4 weeks after, intradermal IPV given at 8 weeks interval Neutralizing antibody titre to poliovirus

Type 1 Subject no.

Pre a

Post s

1 2 3 4 5 6 7 8 9

64 16