Heterogeneities in the transmission of infectious agents: implications for the design of control programs

Abstract

From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R0. This is an example of a statistical pattern known as the 20/80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the "core" 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole.

Figure 1

Effects of discounting hosts (as individuals or as households) on the relative value of R_c (see text). Relative R_c (as a proportion of R₀) is plotted against cumulative fraction discounted (these are step functions over the interval 1/m). The intercept between relative R_c = 0.2 and fraction discounted = 0.2, corresponding to the 20/80 rule, is shown in each case. (a) Vector–host ratios for parasites transmitted by biting arthropods. LB, Leishmania chagasi transmission on Marajo island, Brazil; MP, malaria transmission in Wosera, Papua New Guinea; MT, malaria transmission at Namawala in Tanzania. (b) Water contact rates for parasites transmitted via snails. SM, S. haematobium and S. mansoni transmission at the villages Maniale, Medina Coura, and Dogofry Ba in Mali; SZ, S. haematobium transmission at Nyamikari and Nahoon farms, Zimbabwe. (c) Rates of sexual partner change for STDs: UK, new partners (over 4 years) among the adult population of the United Kingdom; France, new partners (over 1 year) among the adult population of France. This data set includes censored values that were taken as the lower limit of each interval. The different shapes of these curves reflects the different time scales of the observations.

Figure 2

Relationship of reduction in R_c to the distribution of contributions to transmission potential. Frequency distributions for (a) vector–host ratios for Anopheles spp. and humans in Wosera, Papua New Guinea (see Fig. 1a), standardized variance (SV) equals 4.01; (b) contacts with schistosome infective water bodies for individuals in Dogofry Ba, Mali (see Fig. 1b), SV = 2.75; (c) changes of sexual partners for individuals in the United Kingdom (see Fig. 1c), SV = 12.82. (d) Expected effects of discounting host subgroups in decreasing order of vector-to-host ratios (following a log-uniform distribution) on the relative value of R_c for microparasites, compared for values of SV from 0 to 5 (as shown). A value of SV = 1.083 corresponds to the 20/80 rule.