and endemic in some risk groups (such as FSWs) while being below the sustainability threshold in other groups.28 In the latter groups, STIs can still spread, but the transmission chains are not sustainable and eventually face “dead ends.” This variability in risk leads to the group of sustainable transmission (GST)29 concept, where the GST for a speCore group Bridging General population populations (including vulnerable cific STI is the population where the populations) infection’s transmission chains are sustainable (R0 ⬎ 1). The size of the GST in any community determines the potential size of the STI epidemic, and different STIs have different GST sizes (see appendix B, table B.1). HIV spreads most rapidly in the high-risk core populations due to the higher levels of risk behavior, the different risk groups, and size of the implying an R0 considerably larger than one. different risk groups. Therefore, IDUs, MSM, and FSWs are normally The heterogeneity in the spread of HIV or the groups that first experience the burden of other STIs’ spread can be understood analytithe HIV epidemic (figure 1.3). Subsequently, cally in terms of the basic reproductive number (R0): the number of secondary infections that HIV spreads to the bridging populations (sexual an index case would generate upon entrance partners of core populations). Bridging groups into an infection-free population.24 For an infec- may or may not pass the infection to the gention to spread, R0 needs to be larger than one eral population. If bridging populations do pass (figure 1.2, panel A). If R0 ⬍ 1, the number of the infection to the general population, then the secondary infections each index case produces is levels of risk behavior in the latter population on average less than one and the infection would determine the potential for a general cannot sustain itself and dies out (figure 1.2, population HIV epidemic. If the levels of risk are panel B). R0 for a specific population depends on sufficiently high, such that R0 ⬎ 1, then the the risk behavior attributes of this population infectious transmission chains are sustainable and duration of infection, as well as the trans- among the general population irrespective of mission probability per coital act. The last can be further sexual contacts with core and bridging influenced by a number of biological factors populations (figure 1.3, panel A). HIV will such as stage of HIV infection,25 male circumci- remain endemic in the general population even if all sexual contacts with core and bridging sion status,26 and coinfections.27 Since sexual risk behavior and injecting drug populations are protected or stopped. This is use are characterized by substantial heterogene- what has occurred in parts of sub-Saharan ity, each risk group in the population has its Africa, resulting in massive HIV epidemics. On the other hand, if R0 ⬍ 1, the HIV transown R0. HIV or other STIs can be sustainable mission chains are not sustainable, and the waves of HIV spread in the general population 24 Anderson and May, Infectious Diseases of Humans. 25 would stop shortly after entering this population Wawer et al., “Rates of HIV-1 Transmission.”
Figure 1.1 Heterogeneity in Risk of Exposure to a Sexually Transmitted Infection
Source: Authors.
26
Auvert et al., “Randomized, Controlled Intervention Trial”; Bailey et al., “Male Circumcision for HIV Prevention”; Gray et al., “Male Circumcision for HIV Prevention.” 27 Abu-Raddad et al., “Genital Herpes”; Abu-Raddad, Patnaik, and Kublin, “Dual Infection with HIV and Malaria.” 4
Characterizing the HIV/AIDS Epidemic in the Middle East and North Africa
28
Boily and Masse, “Mathematical Models”; Brunham and Plummer, “A General Model”; Yorke, Hethcote, and Nold, “Dynamics and Control.” 29 Abu-Raddad et al., “Genital Herpes”; Boily and Masse, “Mathematical Models”; Brunham and Plummer, “A General Model.”