Population-Level Persistence of Immunity 2 Years After the PsA-TT Mass-Vaccination Campaign in Mali.

Efforts to prevent the large-scale outbreaks of meningococcal disease that have led to significant morbidity and mortality in the African meningitis belt, a region stretching from Senegal to Ethiopia, changed dramatically in 2010 when a newly developed group A meningococcal polysaccharide–tetanus toxoid protein conjugate vaccine, PsA-TT or MenAfriVac, was introduced in the first-ever preventive mass vaccination campaign in Burkina Faso, Mali, and Niger [1–4]. Developed by the Meningitis Vaccine Project, a partnership between the World Health Organization (WHO) and PATH with funding from the Bill & Melinda Gates Foundation, PsA-TT transformed the reactive vaccination strategy in place in the Meningitis Belt since the 1970s, which utilized polysaccharide meningococcal vaccines to respond to meningococcal disease outbreaks, into a preventive campaign that is implementing mass vaccination of individuals aged 1–29 years in >25 countries in Africa [5]. Evidence has indicated that PsA-TT is highly immunogenic; the vaccine was licensed based on immunogenicity alone [2, 6–8]. By the end of 2014, >217 million people in 15 countries had been vaccinated with PsA-TT [9]. The introduction of PsA-TT has dramatically altered the epidemiology of meningococcal disease in Africa, and significant reductions in the incidence of meningococcal A disease [10, 11] and meningococcal A carriage [12, 13] have been achieved in countries where the vaccine has been introduced. As with all recently developed and newly introduced vaccines, questions remain about the duration of protection provided by PsA-TT. Assessing the duration of protection has been identified as a priority for the prevention and control of meningitis in Africa [14]. Evaluating antibody persistence both in the context of clinical trials and in population-based epidemiologic studies is an important component of investigating the impact conjugate meningococcal A vaccination has had on immunity in this region. Meningococcal conjugate vaccines including PsA-TT join together polysaccharides from the outer membrane of Neisseria meningitidis to a protein carrier such as tetanus toxoid. Conjugate vaccines elicit a more robust immune response than polysaccharide-only vaccines, which are limited in that immunity wanes after 3–5 years and in that they are not immunogenic in very young children [15–17]. Although conjugate meningococcal C and ACWY vaccines provide a longer duration of immunity than comparable polysaccharide vaccines, antibody levels have been shown to wane rapidly following vaccination [18, 19]. Booster doses have been added to vaccination schedules in both the United States, where meningococcal ACWY conjugate vaccine is recommended for 11- and 12-year-olds with a booster dose at age 16 years [20], and the United Kingdom, where meningococcal C conjugate vaccine is recommended for infants and booster doses are given at 12 months and around 14 years [21]. Booster doses for these meningococcal conjugate vaccines have been recommended due to concerns about waning immunity during periods of high risk [18, 22–24], further emphasizing the need to understand the duration of immunity following PsA-TT introduction has been introduced in areas where meningococcal disease is highly endemic. To assess the duration of protection and changes in population-level immunity over time following the 2010 introduction of PsA-TT, we established a cohort in 2012 among residents of Bamako, Mali. Here we report the results of the first seroprevalence survey undertaken in the cohort, 2 years after the PsA-TT mass vaccination campaign.