Some bacteria when infecting humans are often protected by a polysaccharide (sugar) capsule that helps the organism evade the human defense systems especially in infants and young children.
Polysaccharide vaccines create a response against the molecules in the pathogen’s capsule. These molecules are small, and often not very immunogenic. As consequence they tend to:
- Not be effective in infants and young children (under 18-24 months)
- Induce only short-term immunity (slow immune response, slow rise of antibody levels, no immune memory)
Examples of polysaccharide vaccines include Meningococcal disease caused by Neisseria meningitidis groups A, C, W135 and Y, as well as Pneumococcal disease.