Cause
The bacterium Neisseria meningitidis, of which 12 serogroups are known. Most cases of meningococcal disease are caused by serogroups A, B and C; less commonly, infection is caused by serogroups Y (emerging in the United States) and W-135 (particularly in Burkina Faso). Some small-scale outbreaks caused by serogroup X have been reported in Niger and Uganda. Epidemics in Africa are usually caused by N. meningitidis type A.
Transmission
Transmission occurs by direct person-to-person contact, and through respiratory droplets from the nose and pharynx of infected persons, patients or asymptomatic carriers. Humans are the only reservoir.
Nature of the disease
Most infections do not cause clinical disease. Many infected people become asymptomatic carriers of the bacteria and serve as a reservoir and source of infection for others. In general, susceptibility to meningococcal disease decreases with age, although there is a small increase in risk in adolescents and young adults. Meningococcal meningitis has a sudden onset of intense headache, fever, nausea, vomiting, photophobia and stiff neck, plus various neurological signs. The disease is fatal in 5–10% of cases even with prompt antimicrobial treatment in good health care facilities; among individuals who survive, up to 20% have permanent neurological sequelae. Meningococcal septicaemia, in which there is rapid dissemination of bacteria in the bloodstream, is a less common form of meningococcal disease, characterized by circulatory collapse, haemorrhagic skin rash and high fatality rate.
Geographical distribution
Sporadic cases are found worldwide. In temperate zones, most cases occur in the winter months. Localized outbreaks occur in enclosed crowded spaces (e.g. dormitories, military barracks). In sub-Saharan Africa, in a zone stretching across the continent from Senegal to Ethiopia (the African “meningitis belt”), large outbreaks and epidemics take place during the dry season (November–June). Recent reports of endemic occurrence of group Y meningococcal disease in the United States, and outbreaks cau73 sed by serogroup W-135 strains in Saudi Arabia and sub-Saharan Africa, particularly Burkina Faso, suggest that these serogroups may be gaining in importance.
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 indigenous population may be at greater risk of infection. In recent years, outbreaks caused by serogroups W135 have also occurred.
- 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.
Vaccine
Polysaccharide vaccines
Internationally marketed meningococcal polysaccharide vaccines are either bivalent (A and C) or tetravalent (A, C, Y and W-135).The vaccines are purifi ed, heatstable, lyophilized capsular polysaccharides from meningococci of the respective serogroups.
Both group A and group C vaccines have documented short-term effi cacy 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 effi cacy 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.
A protective antibody response occurs within 10 days of vaccination. In schoolchildren and adults, the bivalent and tetravalent polysaccharide vaccines appear to provide protection for at least 3 years, but in children under 4 years the levels of specifi c antibodies decline rapidly after 2–3 years.
The currently available bivalent and tetravalent meningococcal vaccines are recommended for immunization of specifi c 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 fi rst 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.
Precautions and contraindications
The internationally available polysaccharide vaccines are safe, and signifi cant systemic reactions have been extremely rare. The most common adverse reactions are erythema and slight pain at the site of injection for 1–2 days. Fever exceeding 38.5 ºC occurs in up to 2% of vaccinees. No signifi cant change in safety or reactogenicity has been observed when the different group-specifi c polysaccharides are combined into bivalent or tetravalent meningococcal vaccines. Cross-protection does not occur and travellers already immunized with conjugate vaccine against serogroup C are not protected against other serogroups.
Type of vaccine: Purified bacterial capsular polysaccharide meningococcal
vaccine (bivalent or tetravalent)
Number of doses: One
Duration of protection: 3–5 years
Contraindications: Serious adverse reaction to previous dose
Adverse reactions: Occasional mild local reactions; rarely, fever
Before departure: 2 weeks
Consider for: All travellers to countries in the sub-Saharan meningitis belt and to areas with current epidemics; college students at risk from endemic disease; Hajj and Umrah pilgrims (mandatory) Special precautions: Children under 2 years of age are not protected by the vaccine