This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
The current vaccine not highly immunogenic in children under 2, or
very long lasting, and it is children under 2 who have the highest
rate of the disease. Research is being done on conjugate vaccines
which would produce a greater immune response. Global cooperation
would be needed to make these affordable in developing
countries. (Williams, 1997)