Science commentary: Why conjugate vaccines protect longerBMJ 1998; 316 doi: http://dx.doi.org/10.1136/bmj.316.7144.1571 (Published 23 May 1998) Cite this as: BMJ 1998;316:1571
The effectiveness of polysaccharide vaccines against diseases such as meningococcal meningitis is limited because they do not stimulate a T cell response, which is required to activate long term immunological memory. Also, polysaccharide vaccines induce poor responses in infants. Up to 40% of cases of meningitis in the United Kingdom are caused by Neisseria meningitidis type c; N meningitidis is particularly virulent in young babies and teenagers. Because the polysaccharide vaccine against meningitis A and C does not induce the production of immunological memory cells and is relatively ineffective in children aged under 2, it does not protect these vulnerable groups.
Conjugate vaccines are being developed to overcome this lack of long term memory. Conjugation involves attaching the required polysaccharide component of the bacteria against which the immune response is to be directed to a protein. This conjugated complex then behaves more like a protein and is presented and processed in a T cell dependent manner. Activated T cells then secrete cytokines that elicit a long term memory response, which includes the production of antibodies and cell mediated immunity. This response occurs at all ages.
Conjugated vaccines against meningitis A and C are likely to undergo clinical trials within two years. Developing a vaccine against meningitis B is harder because the risk of a cross reaction between certain HLA antigens and the B capsule polysaccharide, which could result in an inappropriate autoimmune response.
Conjugate vaccines are also being developed against pneumococcal disease: 11 of the commonest childhood pneumococcal serotypes have been conjugated together with a protein in one vaccine, which should offer long term immunity against otitis media, community acquired pneumonia, septicaemia, and meningitis. This vaccine is being assessed in clinical trials; results are expected in 1999—Abi Berger, Science editor, BMJ.