Two types: polysaccharide and conjugate
Two types of meningococcal vaccines useful in the African epidemiological context are available globally: polysaccharide vaccines and conjugate vaccines.
Limitations of polysaccharide vaccines
Polysaccharide vaccines exist in various combinations against groups A, C, W135, and Y. These vaccines have existed for over 30 years and are highly effective in preventing disease in older children and adults. They are also used in reactive mass vaccination campaigns in Africa. However, these polysaccharide vaccines have several shortcomings that preclude their incorporation into routine immunization programs:
- They do not protect infants.
- They provide protection for only two to three years.
- They do not decrease carriage of the bacteria and do not induce herd immunity (whereby transmission of the bacterium is blocked, thus extending protection to the unvaccinated).
That is why polysaccharide vaccines are mainly used either to control outbreaks or to protect travelers going to countries affected by the disease.
In Africa, repeated vaccination of the broad target age-group is programmatically very difficult. A monovalent group A conjugate vaccine providing longer-term protection at all ages and conferring herd immunity is far better suited for prevention of meningococcal epidemics in Africa.
Advantages of conjugate vaccines
Research shows that conjugating a protein, such as diphtheria or tetanus toxoid, to a polysaccharide antigen results in a vaccine that works better and gives better protection over a long period of time. This conjugation process has been used to develop several meningitis vaccines, including several group C meningococcal vaccines, a Hib vaccine, and a pneumococcal vaccine.
Using the same conjugation technology, a polysaccharide-protein conjugate group A vaccine now exists for use in Africa that should:
- Protect very young children.
- Provide longer-lasting protection than polysaccharide vaccines can.
- Decrease bacterial carriage and induce herd immunity, thereby protecting the unvaccinated.
The conjugation technology for developing an effective conjugate vaccine has existed for over ten years, so why did it take so long to develop a group A conjugate vaccine?