Q. Meningococcal vaccine contains purified lyophilised polysaccharide (carbohydrate) of Neisseria meningitidis A and e. Carbohydrate antigens are 'T -independent', and the immune response to carbohydrate antigens is restricted in a number of ways and is not reliably produced in children under the age of 18 months. Carbohydrate antigens alone thus make relatively poor vaccine constituents and cannot be used successfully for vaccines designed to prevent infections in infants and toddlers. I seek clarifications on the following two points:

1. Why the minimum age recommended for Meningococcal vaccine is 2 years and not 18 months?

2. Why is it recommended even at the age of 3 months onwards if there has been a case of Meningococcal meningitis in the family?

Yash Paul,
A-D-7, Devi Marg,
Bani Park,
Jaipur 302 016.

A. Neisseria meningitidis serogroups A, B, C, X, Y, Z and W -135 are capable of causing invasive disease. Meningococcal vaccines consist of the capsular polysaccharide antigens of different serogroups. Vaccine may be monovalent (group A), bivalent (groups A and C), or quadrivalent (groups A,C,Y and W -135). The group B capsular antigen is very poorly antigenic, hence there is no group B meningococcal vaccine. Now all licensed meningococcal vaccines are unconjugated carbohydrate antigens; we expect improved, conjugated vaccines, including group B some time in the future.

Carbohydrate antigens are "T-independent", they stimulate B lymphocytes directly without being processed by antigen-presenting cells (APC). Immune responses come under the regulation of CD 4 positive T helper lymphocytes only after antigens are presented by APe. Cell mediated immunity and antibody class switch from IgM production to IgG production as well as the persistence of memory cells require the participation and regulation of the T helper cells. These "T -dependent" responses do not occur after carbohydrate antigen stimulation. However, if the carbohydrate is conjugated with a protein moiety, then the combination is handled by APC, thus converting it to the T -dependent state. Onto-genetically, the T -dependent immune responses are developed during fetal growth and are present at birth. T -independent response by the direct stimulation of B cells develop after birth. This function is weak in the neonatal age, it gradually increases and is reasonably developed at about 18 to 24 month of age. It reaches the peak by about 4 to 5 years of age. Carbohydrate antigens also vary in their immunogenic properties, some like meningococcal group A antigen being a better immunogen at 3 months of age than most other antigens. Most T-independent antigens stimulate antibody from 18 months and certainly by 2 years. Again, most antigens induce the best responses at about 4 or 5 years of age. Some like group B antigen is not at all immunogenic to a satisfactory level.

The group A meningococcal vaccine is moderately immunogenic even in infants of 3 months and above. Therefore, during outbreaks of group A disease, it can be given to infants from 3 months. However, two doses, 3 months apart, are recommended if it is given below 18 months. At or after 18 months, only one dose is sufficient against the risk prevalent at that time
.
The group C vaccine is immunogenic from about 18 to 24 months. Therefore C monovalent or A, C bivalent vaccine is recommended at or after 18 months in some regions and at or after 2 years in others. In the United States, the quadrivalent (A,C,Y,W-135) vaccine is licensed to be given at or after 2 years. If the risk of disease is high, it can be given even at 18 months. If any of these vaccines are given at or after 4-5 years of age, the antibody levels achieved are high and sustained for 10 years or more. Below 4 years antibody levels decline faster; therefore, if vaccine had been given at 18 to 24 months, revaccination is recommended at the next season of risk.

T. Jacob John,
Chairman, lAP Immunization Committee,
Thekkekara,
2/91 E2, Kamalakshipuram,
Vellore - 632 002,
India.