Pure polysaccharide vaccines are not consistently immunogenic in children younger than two years. What overcomes this limitation?

1. The response to a pure polysaccharide vaccine is T-cell-independent, stimulating B-cells without help from T-helper cells. 2. Children under two years respond poorly to T-cell-independent antigens, probably because the immune system is still immature. 3. Chemically attaching the polysaccharide to a protein molecule, a process called conjugation, changes that response. 4. Conjugation produces long-lasting protective immunity to the polysaccharide antigen, which is how Haemophilus influenzae type b and pneumococcal conjugate vaccines work. 5. A larger dose does not fix an immature response to that class of antigen. 6. Adjuvants and heat inactivation belong to other vaccine types and are not offered as the fix for polysaccharide antigens. _Source: CDC Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), section Inactivated Vaccines_