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The recent article and letter1,2 discussing recommendations for use of heptavalent pneumococcal conjugate vaccine (Prevenar) for at risk children is timely and interesting. We concur with the authors that further immunogenicity studies are necessary in various high risk groups to demonstrate the best protective schedule.
Children older than 2 years with recurrent infections and normal humoral immunity assessed by serum immunoglobulin levels and specific antibody responses to protein antigens, but repeated poor responses (less than 4-fold rise in antibody titers) to 23 valent pneumococcal polysaccharide vaccine (Pneumovax) using standard pneumovax based ELISA3 are labelled as “specific pneumococcal polysaccharide antibody” deficiency.4
We looked at the immunogenicity of the heptavalent conjugate pneumococcal vaccine (Prevenar) in five children aged 4–12 years with specific polysaccharide antibody deficiency by the above definition. Blood was collected before and 4 weeks after immunisation with the heptavalent conjugate pneumococcal vaccine. Serum was analysed using the standard ELISA (using Pneumovax as the antigen)4 and by the newer serotype specific antibody assay.
Results are shown in table 1 and 2. The standard assay showed 4-fold response in only one child. However 5/5 children showed 4-fold responses to at least four of the seven serotypes using the serotype specific antibody assay.
Protection is assumed at a serotype specific antibody level of 0.2 ug/ml or greater.5 It is interesting to note that 4 out of 5 children had achieved such protective levels to four or more serotypes after immunisation with pneumovax as suggested by the serotype specific assay on the pre-prevnar sera.
Clinicians may be tempted to use the more easily available standard ELISA to assess responses to the conjugate vaccine in high risk children. These findings suggest that whilst assessing such responses it is important to use the serotype specific assay to get true measure of adequacy of response (Balmer P et al. Measurement and interpretation of pneumococcal IgG levels for clinical management. Clin Exp Immun (submitted)). The study also suggests that there may be a group of children in whom an immune response to Pneumovax is detectable only by the serotype specific assay and who may be labelled as specific antibody deficient by the standard assay.