Superspreading and heterogeneity of transmission

In SARS and some other infections, a few individuals seem to account for a disproportionate number of the secondary cases. In Singapore, for example, 162 (81 percent) of probable SARS patients showed no evidence of secondary transmission, whereas 5 persons each transmitted infection to 10 or more others (see Figure 1.5; CDC 2003). Lloyd-Smith and colleagues showed that heterogeneity in transmission, instead of being an anomalous event, was a common feature of disease spread (Lloyd-Smith et al., 2005). In several examples, 20 percent of the cases caused 80 percent of the transmission. This is of more than theoretical interest, as it can affect the course of an epidemic and can influence the choice of interventions to try to curb the spread of infection (Galvani and May, 2005). If highly contagious (or highly connected, for sexually transmitted infections) individuals can be identified, it may be possible to contain spread with fewer resources.

* Patient 1 represents Case 1; Patient 6, Case 2; Patient 35, Case 3; Patient 130, Case 4; and Patient 127, Case 5. Excludes 22 cases with either no or poorly defined direct contacts or who were cases translocated to Singapore and the seven contacts of one of these cases. Reference: Bogatti SP. Netdraw 1.0 Network Visualization Software. Harvard, Massachusetts: Analytic Technologies, 2002.

* Patient 1 represents Case 1; Patient 6, Case 2; Patient 35, Case 3; Patient 130, Case 4; and Patient 127, Case 5. Excludes 22 cases with either no or poorly defined direct contacts or who were cases translocated to Singapore and the seven contacts of one of these cases. Reference: Bogatti SP. Netdraw 1.0 Network Visualization Software. Harvard, Massachusetts: Analytic Technologies, 2002.

Figure 1.5 Superspreaders. Probable SARS cases by reported source of infection, Singapore (25 Feb-30 Apr 2003). From CDC (2003).

A small study that may be relevant to the topic of heterogeneity in transmission of respiratory pathogens assessed exhaled aerosols. Eleven healthy adult volunteers (18-65 years old) were connected to a device that measured the number and size of particles exhaled during quiet breathing. The number of exhaled particles varied dramatically among the subjects, ranging from 1 per liter to >10,000 per liter. Over a 3-month period, the high producers remained high producers on repeat assessment (Edwards et al., 2004).

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