Continuing Effectiveness of Serogroup A Meningococcal Conjugate Vaccine, Chad, 2013

In 2011, vaccination with a serogroup A meningococcal polysaccharide conjugate vaccine was implemented in 3 of 23 regions in Chad. Cases of meningitis declined dramatically in vaccinated areas, but an epidemic continued in the rest of Chad. In 2012, the remaining Chad population was vaccinated, and the epidemic was halted.

In 2011, vaccination with a serogroup A meningococcal polysaccharide conjugate vaccine was implemented in 3 of 23 regions in Chad. Cases of meningitis declined dramatically in vaccinated areas, but an epidemic continued in the rest of Chad. In 2012, the remaining Chad population was vaccinated, and the epidemic was halted.
F or >100 years, countries in the meningitis belt of Africa have experienced intermittent epidemics of meningococcal meningitis, caused mainly by the serogroup A meningococcus (1). After development and prequalification of a new serogroup A meningococcal polysaccharide/tetanus toxoid conjugate vaccine (PsA-TT) in 2009 (2), vaccination with PsA-TT across the meningitis belt commenced in 2010, starting with persons 1-29 years of age in Burkina Faso and parts of Mali and Niger (3). Little transmission of the serogroup A meningococcus was occurring in these countries at the time of vaccine introduction, making evaluation of its effectiveness difficult.
In contrast, in Chad, PsA-TT was introduced in the middle of a serogroup A meningococcal epidemic, and vaccination with PsA-TT commenced at the end of 2011, shortly before the 2012 epidemic season. At this time, vaccination of persons 1-29 years of age (target 1.8 million) was undertaken in the capital N'Djamena, Mayo Kebbi Est, and Chari Baguirmi (4), designated here as the N'Djamena regions ( Figure 1). In 2012, the vaccination program was extended to the rest of the country (target 5.9 million) (Figure 1). During the 2012 meningitis season, the incidence of meningitis decreased by >90% in vaccinated areas compared with the rest of the country, and a similar reduction in the incidence of carriage of serogroup A Neisseria meningitidis was found, as reported previously (4). We report on the incidence of meningitis during the 2013 meningitis season after vaccination of persons 1-29 years of age in areas with no prior vaccination program.

The Study
In Chad, health districts provide aggregated weekly data on meningitis and other notifiable diseases to the Ministry of Health. This system was reinforced in 2012 and 2013 by case-based surveillance supported by the Centre du Support en Santé Internationale in the N'Djamena regions, and also by Médecins sans Frontières in Moissala, a district ≈800 km from NDjamena ( Figure 1). Cerebrospinal fluid (CSF) specimens obtained from persons with suspected cases of meningitis were transported to the national reference laboratory in NDjamena. Isolated strains of N. meningitidis were sent to the World Health Organization (WHO) Intercountry Support Team, Ouagadougou, Burkina Faso, and to the WHO Collaborating Centre for Reference and Research on Meningococci in Oslo. Information about the laboratory methods used to isolate and characterize meningococci is provided elsewhere (4). Data from the 2009 census were used to calculate incidence rates. We used a negative binomial regression model to assess the effect of PsA-TT on the incidence of meningitis in the N'Djamena regions in 2012 and in the whole country in 2013; we used weekly data obtained during the epidemic period (weeks 1-26) during 2009-2013.
The incidence of meningitis in Chad during 2009-2013 and its association with the introduction of PsA-TT are shown in Figure 2. During weeks 1-26 of 2012, the incidence of reported meningitis among persons in all age groups in the N'Djamena regions that received vaccine was 2.5 cases/100,000 population (57/2.3 million); during the previous year, incidence was 31.8/100,000 (732/2. ratio for vaccinated versus unvaccinated populations was estimated by using data across the whole study period with a negative binomial regression model; the incidence rate ratio was 0.104 (95% CI 0.052-0.207).
Fewer CSF specimens were submitted to the national reference laboratory in N'Djamena in 2013 than in 2012 (Table), but the proportion of reported cases for which CSF samples were submitted increased from 8.

Conclusions
We previously reported a >90% reduction in incidence of meningitis among vaccinated populations in Chad in 2011 (4). Here we report a similar reduction in the incidence of meningitis in 2013 from that in 2012 for populations vaccinated only during the second year of the vaccination campaign. The epidemic curve (Figure 2) suggests that by 2013, the N. meningitidis serogroup A epidemic in Chad was waning and that fewer cases of serogroup A meningitis would have occurred during 2013 than during 2012, even in the absence of vaccination. However, the incidence of meningitis dropped lower in 2013 than would have been expected as a result of a natural decline, and only 1 serogroup A isolate was obtained at the national reference laboratory despite improved CSF sampling. This finding provides strong additional evidence of vaccine effectiveness for preventing serogroup A meningococcal disease in Chad.
If the effectiveness of PsA-TT vaccination seen in Chad and Burkina Faso (6,7) is replicated across the meningitis belt of Africa and if vaccine coverage can be sustained through introduction of PsA-TT into the infant immunization program and/or through mass campaigns, serogroup A epidemics could disappear from the meningitis belt. However, past experience has shown that meningococci belonging to serogroups C, W, or X can cause substantial epidemics (8-10); therefore, continuing surveillance will be needed to determine how the epidemiology of meningococcal disease in the meningitis belt of Africa is changed by the successful introduction of PsA-TT (11). The Chad Ministry of Health has approved a plan to support and develop case-based surveillance in the N'Djamena regions, Moissala, and 3 other selected health districts.   Effectiveness of Meningococcal Vaccine, Chad