- Polysaccharide vaccines are a class of vaccines that use purified polysaccharides from the outer capsules of bacteria as antigens to stimulate protective immunity.
- Many pathogenic bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae type b (Hib), Neisseria meningitidis, and Salmonella typhi, possess thick polysaccharide capsules that serve as virulence factors, helping them evade phagocytosis and host immune responses. By isolating and purifying these polysaccharide components, scientists have developed vaccines that target the bacterial capsule, thereby priming the immune system to recognize and combat the pathogen upon exposure.
- Polysaccharide vaccines function by inducing the production of antibodies directed against the capsule polysaccharides. However, unlike protein antigens, polysaccharides are recognized by the immune system in a T-cell independent manner, primarily activating B cells directly. This results in limited immune memory and poor responses in children under two years of age, since their immune systems are not fully mature to mount strong T-independent responses. Moreover, booster doses often do not enhance immunity significantly, as classical polysaccharide vaccines lack strong immunological memory.
- To overcome these limitations, conjugate vaccines were developed, where the polysaccharide is chemically linked to a protein carrier (such as diphtheria or tetanus toxoid). This modification converts the immune response from T-cell independent to T-cell dependent, allowing the generation of long-lasting immunological memory, stronger antibody production, and effectiveness in infants and young children. Conjugate vaccines have revolutionized the prevention of bacterial diseases: Hib conjugate vaccines, for instance, have nearly eliminated invasive Hib infections in countries with high vaccine coverage.
- Clinically, polysaccharide and polysaccharide-protein conjugate vaccines are used to prevent several life-threatening infections. Pneumococcal vaccines include both polysaccharide-only formulations (PPSV23, targeting 23 S. pneumoniae serotypes) and conjugate formulations (PCV13, targeting 13 serotypes). Meningococcal vaccines target the capsule polysaccharides of Neisseria meningitidis serogroups A, C, W, and Y, with conjugated versions now preferred in children and adolescents. Similarly, Hib conjugate vaccines have dramatically reduced global rates of meningitis and pneumonia caused by H. influenzae type b. Typhoid Vi polysaccharide vaccines also serve as an example, though conjugated forms are increasingly adopted for broader and longer-lasting protection.
- From a public health perspective, polysaccharide-based vaccines have played a transformative role in reducing the burden of bacterial meningitis, pneumonia, sepsis, and bloodstream infections. The advent of conjugate technology has extended their protection to vulnerable populations, including infants and immunocompromised individuals, making them central tools in global immunization programs.
