Basic reproductive rate

Whether infection can be sustained and will spread in a new population depends on the basic reproductive rate (R0), which is the average number of secondary infections produced by an infected individual in a susceptible population (Anderson and May, 1991). For an infection to be sustained in a new population, the basic reproductive rate must exceed one (May et al., 2001). The R0 for a specific infection may vary depending on time, place, and population. For example, in the pre-vaccine era the R0 for measles was 16-18 in England and Wales in 1950-68, and 5-6 in Kansas, USA, in 1918-21. A plausible explanation for the difference is differences in population density and mixing of populations. In the early 1980s, the R0 for HIV was 11-12 in Nairobi, Kenya, in a prostitute population and 2-5 in male homosexuals in England and Wales (Anderson and May, 1991). Based on detailed epidemiologic data from Singapore and other locations, Lipsitch and colleagues estimated that the reproductive number for SARS was about 3 (Lipsitch et al., 2003). SARS was barely containable using isolation and quarantine measures. One biological characteristic of SARS that favored its control was that virtually all transmission occurred after patients had developed symptoms. This is not the case for seasonal influenza, HIV, and many other infections that can be transmitted in the absence of symptoms (Fraser et al., 2004).

Multiple factors can influence the R0, including population density (noted above), behavior, genetics of the host population, and evolution of the pathogen. If a population includes a large number of hosts who are immunocompromised and remain infected and infectious for a prolonged period, this could also influence R0. A pathogen with a high mutation rate that allows it to adapt more readily to new hosts is more likely to be successful in emergence (Antia et al., 2003).

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