Neisseria meningitidis or meningococcus is a diplococcus Gram-negative bacterium that is known to colonize the human nasopharynx of approximately 10% of the population at any given time.1 Transmission occurs via the inhalation of respiratory droplets or through direct contact with nasopharynx secretions from asymptomatic or symptomatic carriers.2 Occasionally, the pathogen invades the mucosa and enters the bloodstream to cause invasive meningococcal disease (IMD), such as meningitis or septicemia.3,4 Less common manifestations of meningococcal disease include myocarditis, endocarditis and pericarditis.4

N. meningitidis strains are categorized into 12 capsular types (A, B, C, E, H, I, K, L, W, X, Y and Z), according to the biochemical composition and structure of capsular polysaccharide.5 Six serogroups (A, B, C, W, X, and Y) are responsible for the majority of IMD cases, and serogroup prevalence varies temporarily and by geographic location.5,6

Meningococcal colonization is the initial step in the development of IMD.3 As a consequence, since the middle of the 20th century, studies have been carried out worldwide to analyze the prevalence of asymptomatic carriers of N. meningitidis in order to better understand the transmission process, the epidemiology of this disease and to obtain information to improve vaccination strategies. On the other hand, differences in carrier profiles throughout the American continent, both in terms of prevalence and the population(s) affected, can interfere with the correlation of existing data.

Currently, three types of meningococcal vaccine exist: polysaccharide vaccine, conjugate vaccine, and multi-peptide vaccine.7,8 The polysaccharide vaccine, which is composed of capsule antigens, is falling into disuse due to low immunogenicity.9 The conjugate vaccine, developed by the conjugation of polysaccharide antigens with carrier proteins, may exert an effect known as herd protection, which promotes defense against transmission.10 The multi-peptide vaccine, produced from outer membrane vesicles and subcapsular proteins, was developed to respond to epidemics mainly arising from serogroup B.11

Investigations into meningococcal carriage are crucial to furthering the understanding of transmission dynamics and epidemiology, as well as the potential effects of control programs, such as vaccination.12 At the time of this manuscript elaboration, no article reviews focusing on meningococcal carriage studies in the Americas were found in the literature. The compilation of these studies could provide insight into the epidemiology of meningococcal disease throughout this region, and could assist in the development of targeted strategies to reduce the transmission of specific meningococcal strains. Therefore, the aim of this systematic review was to provide information regarding the prevalence of N. meningitidis carriers in the Americas, emphasizing the risk factors associated with carrier status and the most prevalent serogroups in each studied population.

A systematic review was conducted to identify the prevalence of meningococcal carriage in studies performed in countries located throughout the Americas. The proposed question was: What is the prevalence and associated risk factors for N. meningitidis carriage among people living in American countries? This study was carried out in accordance with the recommendations established by the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes—PRISMA,13 and was also registered in the PROSPERO database under number CRD42018106755.

To the best of our knowledge, this systematic review was the first attempt to describe the prevalence of asymptomatic carriers of N. meningitidis throughout the Americas. The majority (78.3%; n = 18/23) of the populations studied consisted of secondary level students and undergraduates, including 19-year-olds, which is the age considered to represent the peak of carriage.38 It was also observed that carriage prevalence varied among countries, both on the same subcontinent and among different subcontinents, as shown in Fig. 2.

Fig. 2.

Carriage prevalence in the American continent.

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Several studies included semi-closed populations as the object of study, such as undergraduate students,17,19,21,26–28,30,35–37 which are considered groups presenting a high-risk of carriage.39,40

It is important to point out that the variable carrier rates and risk factors identified among selected population groups may not be reflective of the overall situation in the local populations studied.1 For example, studies conducted during outbreaks26,27,29,33 may have impacted their respective reported prevalence rates.

Articles focusing on students presented high rates of carriage prevalence,17,19,20,21,26,27 corroborating studies with similar designs carried out in other countries (10.4% in Greece41 and 12.1% in Italy).42 In fact, adolescents and young adults have been reported as the highest risk group for the acquisition and transmission of N. meningitidis.39 As expected, male sex was associated with carriage in several studies throughout the Americas.18,19,21,24,26–29,32,37 Furthermore, according to other studies conducted worldwide, several other risk factors, such as current exposure to cigarette smoke, attendance at parties and bars, household agglomeration condition, upper respiratory infection, oral sex, were also observed to increase the odds of being a carrier in adolescents and young adults.21,23,25–27,37,40,43

In Brazil, due to the increased prevalence of meningococcal C disease cases, the meningococcal serogroup C conjugate vaccine (MCC) was introduced into the routine infant vaccine schedule in 2010. This vaccine is administered at three and five months, with a booster dose scheduled at 12 months of age.44 The introduction of MCC has greatly contributed to invasive meningococcal disease control in several countries, leading to consequent effects in the carriage and transmission of meningococcal C. These effects can be attributed to both high vaccine effectiveness (direct protection), as well as to herd protection (indirect protection).10,45 Particularly in Salvador, Brazil, the low prevalence of meningococcal C (MenC) carriage observed among individuals aged 11–19 years in 2014 was associated with a catch-up campaign in 2010 that targeted adolescents and young adults in this city.15 The resulting prevalence found in Salvador was around 4–5% lower than that reported by other studies conducted in two other Brazilian cities, which did not implement a booster vaccination campaign in older age-groups.31–34

Reduced meningococcal carriage as a consequence of vaccination campaigns directed at specific serogroups was observed in a study conducted in Cuba.18 The vaccine against serogroups B and C (VA-MENGOC-BC®) was implemented in the National Immunization Program in 1991,46 which could explain the low prevalence of MenC and a high rate of non-groupable strains in subsequent studies.16,17,18,19,20 On the other hand, a significant rate of meningococcal B isolates were detected among the studies conducted in Cuba, and one study reported a higher prevalence than expected in this serogroup, which could be explained by the work activities of the population studied, involved in the manipulation of meningococcal B (MenB) strains.18

In North America, most studies were conducted in the United States to evaluate carriage prevalence and the impact of vaccination on N. meningitidis transmission and colonization.24,26,29,30 Similar to studies performed in other countries, a high prevalence of non-groupable and a low prevalence of groupable strains was found. In addition, some studies evaluating populations vaccinated against MenB and serogroups A, C, W and Y (MenACWY) were unable to determine whether meningococcal carriage and/or acquisition was reduced.21,23,26,27 This stands in contrast to the effectiveness derived from MenC vaccines reported in studies conducted in the UK and Brazil, for example.10,44,45

Some differences with respect to the methodology utilized in the included studies were observed, which seem to justify the lack of comprehensive information found in the articles assessing meningococcal carriage. We identified few studies involving individuals of more advanced age in the Americas, who are also frequently affected by IMD; hence, more studies encompassing this age group are needed. In contrast, most of the studies evaluated herein employed convenience sampling as opposed to a randomized selection method, which often unreliably represents the studied population. Although this may have interfered with the results presented by these studies, their findings nonetheless corroborated the data in the literature as a whole, which diminishes the probability of bias due to sample selection procedures.

This review is limited by the fact that searches were conducted in just four databases, and only articles were included; therefore, some papers published in annals and other sources may not have been included.

Although most of the studies included herein were determined to have a medium risk of bias, the present review attempted to provide a general overview of serogroup distribution correlated with different vaccination programs specific to each country, as well as to identify relevant risk factors among N. meningitidis carriers throughout the Americas. This research was designed to support epidemiological surveillance and prevention measures to control the transmission of meningococci in a variety of populations and countries.

The authors declare no conflicts of interest.