(1) Both versions of the vaccine against Streptococcus pneumoniae contain the capsular polysaccharide of the bacteria as the immunogen. One version contains the capsular polysaccharide of the 23 most prevalent serotypes. It is recommended for persons older than 60 years of age and adult patients of any age with such chronic diseases as diabetes and cirrhosis or with compromised spleen function or splenectomy. A second version containing the capsular polysaccharide of 13 pneumococcal serotypes coupled to a carrier protein (diphtheria toxoid) is available for the protection of young children who do not respond well to the unconjugated vaccine.
A potential problem regarding the use of the pneumococcal vaccine (or a vaccine against any organism with multiple serotypes) is that of serotype replacement. Will the vaccine reduce the incidence of disease caused by the serotypes in the vaccine but not the overall incidence of disease because other serotypes that are not in the vaccine will now cause disease? In fact, this occurred. An increase in invasive pneumococcal disease caused by serotype 19A, a serotype not in the previous vaccine, was observed. In view of this, serotype 19A is now included in the current 13 serotype vaccine.
(2) There are more than a dozen serogroups of Neisseria meningitidis that are classified according to the capsular polysaccharide. Serogroups A, B, C, W, and Y are the primary causes of meningococcal disease throughout the world. In the United States, two quadrivalent meningococcal polysaccharide vaccines conjugated to either diphtheria toxoid or tetanus toxoid are available that protect against serogroups A, C, W, and Y. Immunization with conjugate vaccines elicits a much more robust antibody response, particularly in children, than vaccine formulations containing only polysaccharide as the main antigen.
The capsule of serotype B strains is composed of a polysaccharide that is also found on the surface of many host tissues, so it is a poorly immunogenic antigen, and antibodies raised against the polysaccharide could potentially elicit an autoimmune response by cross-reacting with host cell surfaces. A non polysaccharide vaccine against serogroup B was developed and licensed in 2014, which has dramatically reduced this incidence of bacterial meningitis due to this strain.
Haemophilus influenzae vaccine contains the type b polysaccharide conjugated to diphtheria toxoid or other carrier protein, and is given to children between the ages of 2 and 15 months to prevent meningitis. The capsular polysaccharide alone is a poor immunogen in young children, but coupling it to a carrier protein greatly enhances its immunogenicity.
(3) One of the vaccines against typhoid fever contains the capsular polysaccharide of Salmonella typhi. It is indicated for persons living or traveling in areas where there is a high risk of typhoid fever and for persons in close contact with either infected patients or chronic carriers.
