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Sexual network structure and the spread of HIV in Africa: evidence from Likoma Island, Malawi
Global Health Sciences Literature Digest
Published April 17, 2008
Journal Article

Helleringer S, Kohler HP. Sexual network structure and the spread of HIV in Africa: evidence from Likoma Island, Malawi. AIDS 2007 Nov 12;21(17):2323-32.


To describe the structure of sexual networks among the young adult population of Likoma Island, Malawi, and to identify the distribution of HIV-positive individuals within the sexual network.

Study Design

A cross-sectional observational survey.


Seven geographically contiguous villages on Likoma Island in Lake Malawi. The island is 18 square kilometers, has limited transportation to the mainland, and has a population of slightly more than 7 000 people disbursed in 12 villages.


923 residents of Likoma Island aged 18-35 years


The data collection for the study occurred in two stages. First, a census of all inhabitants on Likoma Island was conducted to obtain a roster of potential sexual partners for the young adult population. Second, the sexual network survey was conducted with all inhabitants 18-35 years of age in seven geographically contiguous villages. Participants were interviewed using audio computer-assisted self-interview (ACASI). Participants were asked to name up to five romantic partners in the previous 3 years and to provide addresses for those located on Likoma Island. For each reported relationship, the participant was asked if it was sexual; the type of relationship (stable, infrequent, or one-off); and whether the relationship was current or had ended more than 1 year ago, within the past year, or within the past month. They were asked the location and circumstances surrounding the initial encounter, whether intercourse occurred, frequency of intercourse, condom use, and symptoms of sexually transmitted infections (STIs). Sex with older/younger partners and partners outside of Likoma also were ascertained, as well as HIV testing history and whether they had received an injection in the previous year. In six of the seven villages, participants were offered HIV testing. Sexual partners were linked using the roster created from the census data.

Primary Outcomes

The structure of the sexual networks among the young adult population of the island.


Of the 923 participants 501 were women. Participation rate varied between villages (range 82%-93%). Respondents in six villages were tested for HIV (n=597) corresponding to participation rates of 54%-68% in the villages. Men were somewhat older than women (mean 26 and 24.6 years, respectively). HIV prevalence was 8% overall and markedly higher among women than among men (10.6%, 95% confidence interval [CI] 7.2-13.9 and 4.7%, 95% CI 2.1-7.3, respectively).

There were 2 040 relationships reported: 1 858 (91%) were sexual, 1 333 (72%) of which were with partners who resided on the island. Of these 1 333 relationships, 845 occurred among persons who also were interviewed as part of the sexual network survey. Fifty-eight percent of the relationships were reported by both of the partners, demonstrating high reliability of the data. Sexual networks were diagramed and included 923 persons from the sexual network survey and 880 who were not in the survey. Of these 1 803 individuals, 1 614 were connected by unique sexual relationships; 244 relationships among survey members were reported by both partners.

Although the average number of partners reported by men (2.6) was higher than the average number reported by women (2.2), the interquartile range was the same (1-3). Five or more partners in the 3 years prior to the survey were reported by 6.7% of women and by 12.6% of men. Marital relationships were stable, meaning that regardless of when they married, couples were still together at the time of the survey.

Separate networks were constructed for partnerships in the past 3 years, the past year, and at the time of the survey. The sexual network of partnerships that occurred in the past 3 years included 883 network members. Within this network were three distinct biocomponents (i.e., subsets of individuals who are connected sexually by two or more independent paths providing a robust network for transmission of infectious diseases).There were two 'diamond'-shaped components linking 4 individuals through various paths and a third very large biocomponent that linked 274 individuals (15% of all members of this network and 25% of all survey respondents). Members within this large biocomponent had on average, 3 partners in the past 3 years and 40% of the members in this group had only 1 or 2 partners. The sexual networks developed from partnerships reported in the past year contained biocomponents that linked 84 partners in the network. The network constructed of partnerships at the time of the survey identified two diamond-shaped biocomponents, each linking 4 individuals. The linear relationships reported by participants occurred at the time of the survey, in the year prior to the survey, and in the 3 years before the survey. HIV prevalence was higher in the sparser, outlying regions of the networks and lower in the biocomponents.


The authors conclude that the sexual networks observed on Likoma Island are compatible with widespread sexual transmission of infectious diseases into low-risk groups. This transmission occurs not through having large numbers of sexual partners but through the interconnectivity of the partnerships. Examination of the demographic characteristics of network members is important because HIV is not uniformly distributed; a network that includes a large proportion of persons with higher HIV prevalence from factors other than the network will contribute to the dynamics of HIV transmission.

Quality Rating

The title and abstract were informative and provided a balanced summary of the study methods, outcomes, and conclusions. However, the description of the study design used an uncommon term. The methods section referred to a manuscript that provides the methods in greater detail; the key elements of the design were poorly organized. The rationale for selection of villages was included but late in the methods section. The variables collected in the census and the results were not described in sufficient detail; the outcomes for the variables were incompletely described. However, the sources for obtaining the data were included. The sample size and statistical methods were not addressed. Participation rate in the census was not described and participation rate in the sexual network survey and HIV testing was included only in a map of the island. Descriptive data are presented only for the survey, without data from the census to place this in context of the island population. Outcome data are not completely described and missing data are not addressed. The main results are described but not well organized. A key figure has descriptive elements that are too small to distinguish from one another, limiting the ability to visualize the key findings of the study. Also, although the paper includes the definitions used to describe sexual networks, including these in the figure would have assisted in its interpretation. The discussion nicely summarized the key findings, discussed limitations, gave a conservative interpretation of the findings, and addressed generalizability.

In Context

This is a novel study which took advantage of a geographically isolated population of sexually active young adults. The use of an island population allowed completing a census that could be used to link and interview sexual partners. Once the partnerships were characterized, the networks could be described. Sexual network analysis primarily has focused on transmission from core groups to bridge groups, such as from bisexual men to women and from sex workers to clients to wives.(1,2,3,4,5) Studies of STI transmission through multiple interconnected chains of partners have been demonstrated in core transmission groups(2,6) and in theoretical models.(5) This paper is the first to do so in a general population in sub-Saharan Africa, one of the few areas of the world with a generalized HIV epidemic. Perhaps the most important finding of this study is the evidence for HIV transmission into a general, low-risk population through the connectivity of the partnerships, rather than through core transmission groups. These data may explain much about the epidemiology of HIV in the region.

Programmatic Implications

The findings of this study demonstrate that complex sexual networks occur among the general population of young adults in a relatively closed community in southern Africa. In contrast to other studies of sexual networks and transmission of pathogens, this study shows that core transmitters are not necessary to result in a large number of persons being sexually connected. These findings offer a possible explanation for the generalized HIV epidemic in southern Africa. They point to the need for better understanding of sexual behavioral norms in this region so that HIV prevention efforts can more effectively help people to reduce the number of their sexual partners, to avoid assuming faithfulness in partners, and to take appropriate precautions.


  1. Ghani A, Ison C, Ward H, Garnett G, Bell G, Kinghorn G, et al. Sexual partner networks in the transmission of sexually transmitted diseases: an analysis of gonorrhea cases in Sheffield, UK. Sex Transm Dis 1996; 23:498-503.
  2. Rothenberg RB, Potterat JJ, Woodhouse DE, Muth SQ, Darrow WW, Klovdahl AS. Social network dynamics and HIV transmission. AIDS 1998; 12:1529-1536.
  3. Wylie J, Cabral T, Jolly A. Identification of networks of sexually transmitted infection: a molecular, geographic, and social network analysis. J Infect Dis 2005; 191:899-906.
  4. Bearman PS, Moody J, Stovel K. Chains of affection: the structure of adolescent romantic and sexual networks. Am J Sociol 2004; 110:44-91.
  5. Newman M. Spread of epidemic disease on networks. Phys Rev E 2002; 66:16128-16211.
  6. Potterat JJ, Phillips-Plummer L, Muth SQ, Rothenberg RB, Woodhouse DE, Maldonado-Long TS, et al. Risk network structure in the early epidemic phase of HIV transmission in Colorado Springs. Sex Transm Infect 2002; 78(Suppl 1):159-163.