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Children are able to respond to protein antigens before they can respond to polysaccharide antigens. The response to pneumococcal polysaccharide vaccine is poor before the age of 2 years and some older children with recurrent respiratory infection also respond poorly to the 23-valent pneumococcal polysaccharide vaccine. Protein conjugate vaccines have been shown to induce significant antibody responses in young infants and now researchers in New Orleans, USA (Ricardo U Sorensen and colleagues, Pediatric Infectious Disease Journal 1998;17:685–91) have shown response to a heptavalent conjugate vaccine in children who did not respond to the polysaccharide vaccine.
They studied 95 children aged 2–13 years who had been referred because of recurrent respiratory infections and who did not have immunoglobulin or IgG subclass deficiencies. After a single dose of 23-valent pneumococcal polysaccharide vaccine the subjects were divided into three groups on the basis of IgG antibody response to nine pneumococcal serotypes: 67 children had an adequate response to five or more of the nine serotypes and they were not offered further immunisation; 111 responded to fewer than five serotypes but did respond to four or more of the serotypes also present in the protein conjugate vaccine and they were given a second dose of polysaccharide vaccine; 17 children responded to fewer than five serotypes and were unresponsive to four or more of the conjugate vaccine serotypes and they were given the experimental heptavalent conjugate vaccine that had as the carrier protein CRM197, a non-toxic variant of diphtheria toxin. In the group given a second dose of polysaccharide vaccine there was no significant increase in antibody concentration to any of the nine serotypes tested. In the group given the conjugate vaccine mean antibody concentration increased for all seven vaccine serotypes although for each serotype there were some patients who failed to respond.
Failure to respond to the 23-valent polysaccharide vaccine is fairly common in children with recurrent respiratory infections (5–10% of such patients in the New Orleans paediatric allergy/immunology clinic). Conjugate vaccines offer hope of treatment but they are not yet available for clinical use.
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