In epidemiology, the speed at which an infectious agent spreads is measured by its reproductive number, ‘R’ – the average number of new cases created by infectious individuals exposed to a susceptible population. When R is greater than 1, the chain of transmission is sustained as each primary case produces at least one secondary case. At the start of an outbreak, the rate is called ‘R0′ (the basic reproductive number). R0 indicates whether or not a contagious disease has the potential to become an epidemic. Populations can evolve natural immunity or gain artificial protection through health interventions like vaccination, reducing the proportion of susceptible people, so the reproductive number for later periods of time is ‘Rt’. R0 was 1.7 to 2 during the initial period of exponential growth in West Africa, while the current Rt is 1.4-1.8 [1]. (Calculations by the WHO Ebola Response Team are roughly in line with estimates by other researchers.) R = 2 doesn’t sound high until you hear that the deadliest pandemic in recorded history, the 1918 Spanish flu, killed up to 100 million people even though the influenza virus had an R of 2. If public health authorities can lower the reproductive number of Ebola below the critical value of 1, the current epidemic will eventually fizzle-out. forbes/sites/jvchamary/2014/10/13/ebola-trends/
Posted on: Wed, 15 Oct 2014 07:15:05 +0000
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