Immune system polysaccharide vaccines

Conjugate vaccines combine toxoids with polysaccharides and attempt to train the immune system to recognize the polysaccharides as being foreign. Young children cannot react to pathogens covered with polysaccharide and Hib conjugates have lowered the infection rate in children from 1 60 to 1 100,000, a worthwhile achievement. 

An important distinction must be made between the humoral response to a pure, capsular polysaccharide, and to the same polysaccharide when it is an integral part of the bacterium. Thus, the immunity received on recovery from infection by encapsulated bacteria, in terms of the polysaccharide antigen, differs from that generated by purposeful immunization with purified capsular-polysaccharide vaccines. Fortunately, with the exception of infants, the polysaccharide vaccines still stimulate protective-antibody levels in humans, despite these differences. In infants, due to the immature nature of their immune systems, these polysaccharide vaccines are of only marginal benefit.7 Some insights into the nature of these different responses in humans can be found in studies on the cellular basis of the immune response to polysaccharides. However, for the purposes of this Chapter, it would be inappropriate to provide a lengthy description of this incompletely understood mechanism in-depth reviews of this burgeoning field of research can be referred to.144-147,162-166... 

In another clinical trial the immunogenicity and safety of polysaccharide vaccine has been assessed in 21 renal transplant recipients (15). Protective antibody titers were reached at 6 and 12 weeks after immunization in aU recipients, bar one. No local or systemic adverse effects were observed. 

Arthus reactions and systemic reactions have commonly been reported after booster doses of polysaccharide vaccine and are thought to result from antigen-antibody reactions involving antibodies induced by the previous immunization (16). Data on revaccination of children are not yet sufficient to provide a basis for recommendation. 

Prevnar is a glycoconjugate heptavalent pneumococcal vaccine for young children made from the polysaccharide capsule of the pneumococcus (20). Although older vaccines made from pneumococcus capsular polysaccharide are available for adults, young children cannot make antibodies to the capsule. By coupling the pneumococcal capsular polysaccharide with a protein carrier, a vaccine was created that will trigger an infant s immune system to produce antibodies. 

Data on adverse events reported to a passive provincial surveillance system have been evaluated after the mass immunization with a polysaccharide meningococcal vaccine of 1 198 751 people aged 6 months to 20 years in Quebec. A total of 118 reports of severe adverse events were selected. The most frequent were allergic reactions (9.2 per 100 000 doses), followed by few neurological reactions (0.5 per 100 000 doses) and very few anaphylactic reactions (0.1 per 100 000 doses). There were no reports of long-lasting sequelae or of encephalopathy, encephalitis, or meningitis (15). 

Pneumococcal vaccines produced by different manufacturers are currently available, for example Pneumovax 23 produced by Merck Sharp Dohme and Pnu-Imune 23 produced by Lederle Laboratories. Each vaccine dose (0.5 ml) contains 25 pg of each polysaccharide antigen. Immunization is recommended for people who are at increased risk of developing pneumococcal disease because of underlying chronic health conditions and for older people. About 50% of vaccinees develop mild adverse effects, such as erythema and pain at the injection site. Fever, myalgia, and severe local reactions have been reported in under 1% of vaccinees. Severe systemic reactions, such as anaphylaxis have been rarely reported. 

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