How infectious is coronavirus?

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A widely shared Twitter thread by Eric Feigl-Ding, a Harvard University epidemiologist, suggests the new coronavirus is "thermonuclear pandemic level bad" based on a metric known as the "r nought" (R0) value. This metric helps determine the basic reproduction number of an infectious disease. In the simplest terms, the value relates to how many people can be infected by one person carrying the disease. It was widely criticized before being deleted.
Infectious diseases such as measles have an R0 of 12 to 18, which is remarkably high. The SARS epidemic of 2002-2003 had an R0 of around 3. A handful of studies modelling the COVID-19 outbreak have given a similar value with a range between 1.4 and 3.8. However, there is a large variation between studies and models attempting to predict the R0 of novel coronavirus due to the constantly changing number of cases. It seems to have settled on a figure around 2.2. 
In the early stages of understanding the disease and its spread, it should be stressed these studies are informative, but they aren't definitive. They give an indication of the potential for the disease to move from person-to-person, but we still don't have enough information about how the new virus spreads. 
"Some experts are saying it is the most infectious virus ever seen -- that is not correct," MacIntyre said. "If it was highly infectious (more infectious than influenza as suggested by some) we should have seen hundreds, if not thousands of cases outside of China by now, given Wuhan is a major travel hub."
China has suggested the virus can spread before symptoms present. Writing in The Conversation on Jan. 28, MacIntyre noted there was no evidence for these claims so far but does suggest children and young people could be infectious without displaying any symptoms. This also makes airport screening less impactful, because harbouring the disease but showing no signs could allow it insidiously spread further.

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