What is a characteristic of pure polysaccharide vaccines?

Pure polysaccharide vaccines are characterized by their inability to activate T helper cells, which in turn means that they do not stimulate B cells through a T-dependent mechanism. These vaccines primarily induce a T-independent immune response, which is less effective in certain populations, especially in young children. Consequently, this lack of T cell activation leads to a weaker immune response compared to conjugate vaccines, which pair polysaccharides with proteins to enhance the immunogenicity, particularly in younger patients.

The other options present characteristics that are not typical of pure polysaccharide vaccines. For instance, pure polysaccharide vaccines may not be universally effective across all age groups, particularly in young children, which counteracts the first option. The third option regarding the need for multiple doses is also not accurate, as these vaccines typically do not require multiple doses to achieve immunity, although certain populations may need boosters due to reduced memory response. As for the last option, pure polysaccharide vaccines do not produce immediate immunity since they often require a period for the body to mount an immune response, thus highlighting the importance of understanding how these vaccines function within the immune system.