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| MENINGOCOCCAL DISEASE |
Disease
Neisseria meningitis is the etiologic agent of two life-threatening disease: meningococcal meningitis and fulminant meningococcemia. More rarely, meningococcal causes pneuomonia, septic arthritis, pericarditits, urethritis and conjunctivitis. Most cases are potentially preventable by vaccination. The disease is transmitted by droplet infection. Up to 40% of the persons are nasopharyngeal carriers of meningococcal, but disease develops in relatively few of these persons. Disease is characterized by high fever, chills, headache, backache, abdominal and extremity pains, nausea and vomiting being typical features. Rapidly developing confusion, delirium,scizures and coma occur in some.
Occurrence
Disease attack rates are highest among infants. Attack rates are higher among children then among adults and there is a second peak of incidence among teenagers in whom outbreaks have been tied to residence in barracks, dormitories or other crowded conditions. This observation has prompted the recommendation that the meningococcal vaccine be administered to incoming college fresh man.
Risk for travellers
Vaccination should be considered for travellers to countries where outbreaks of meningococcal disease are known to occur.
- Travellers to industrialized countries are exposed to the possibility of sporadic cases. Outbreaks of meningococcal C disease occur in schools, colleges, military barracks and other places where large numbers of adolescents and young adults congregate.
- Travellers to the sub-Saharan meningitis belt may be exposed to outbreaks of serogroup A disease with comparatively very high incidence rates during the dry season (December–June). Long-term travellers living in close contact with the indiginous population may be at greater risk of infection.
- Pilgrims to Mecca are at risk. The tetravalent vaccine, (A, C, Y, W-135) is currently required by Saudi Arabia for pilgrims visiting Mecca for the Hajj (annual pilgrimage) or for the Umrah. Outbreaks of meningococcal disease have affected these pilgrims in the past, involving serogroup A in 1987 and both serogroups A and W135 more recently.
Polysaccharide vaccine
Internationally marketed meningococcal polysaccharide vaccines are either bivalent (A and C) or tetravalent (A, C, Y and W-135).The vaccines are purified, heat stable, lyophilized capsular polysaccharides from meningococci of the respective serogroups. Both group A and group C vaccines have documented short-term efficacy levels of 85–100% in older children and adults. However, group C vaccines do not prevent disease in children under 2 years of age, and the efficacy of group A vaccine in children under 1 year of age is unclear. Group Y and W-135 polysaccharides have been shown to be immunogenic only in children over 2 years of age.
Vaccine is given as a single dose subcutaneously and the protective antibodies response occurs within 10-14 days of vaccination. In school-children and adults, the bivalent and tetravalent polysaccharide vaccines appear to provide protection between 3-5 years, but in children under 4 years the levels of specific antibodies decline rapidly after 2–3 years. Thus it is advisable to give two doses of vaccine at an interval of three months to children below the age of two years. A booster dose is given after five years in each group to maintain immunity.
The currently available bivalent and tetravalent meningococcal vaccines are recommended for immunization of specific risk groups as well as for large-scale immunization, as appropriate, for the control of meningococcal outbreaks caused by vaccine-preventable serogroups (A and C, or A, C, Y, W-135 respectively).
Travellers who have access to the tetravalent polysaccharide vaccine (A, C, Y, W-135) should opt for this rather than the bivalent vaccine because of the additional protection against groups Y and W-135.
These vaccines do not provide any protection against group B meningococci, which are the leading cause of endemic meningococcal disease in some countries.
Conjugate vaccines
A T-cell-dependent immune response is achieved through conjugation of the polysaccharide to a protein carrier. Conjugate vaccines are therefore associated with an increased immunogenicity among infants and prolonged duration of protection. Monovalent serogroup C conjugate vaccines were first licensed for use in 1999 and are now incorporated in national vaccination programmes in an increasing number of countries. In contrast to group C polysaccharide vaccines, the group C conjugate vaccine elicits adequate antibody responses and immunological memory even in infants who are vaccinated at 2, 3 and 4 months of age.
More recently, a tetravalent conjugate vaccine (A, C, Y, W-135) has been licensed in a limited number of countries.
The above mentioned vaccines are given subcutaneously as a single dose only.
In one new approach vaccinology is being used to identify outer memberane proteins (OMPs) that are common to all meningococcal strains and serogroups and that may be universal vaccinogen candidate. So far, only few promising candidates have been identified and are ready to undergo clinical trials. |
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