Information from a variety of sources suggests that this novel virus is a recombinant betacoronavirus of animal origin that emerged in November or December 2019, likely at the Wuhan Seafood Market. Epidemiological analysis was initiated after recognition of a market-linked pneumonia cluster in late December. Notwithstanding the name of the “Wuhan Seafood Market”, the market sells large numbers of live animals, including wild animals, which are kept in close proximity to one another, perhaps facilitating viral recombination. Similar disrupted ecology contributed to the emergence of SARS.
The emergence of many cases of a novel, animal-derived pathogen in a live animal market, over a short time period was suggestive of a point source outbreak with animal-to-human spread,assuming that the initial cluster of approximately 40 cases was largely a result of such transmission, with little human-to-human transmission. However, on [23 Jan 2020], the WHO released the report of its IHR Emergency Committee for nCoV; the report noted that “4th generation transmission” was occurring. Several estimates of R0 appeared from independent groups around the same time; these estimates were remarkable in their consistency, ranging from 1.4 to 3.8 (refs. 2-7).
Such consistency despite limited data availability and disparate methods employed for estimation provides a degree of face validity to these estimates. I note that these estimates are likely skewed upwards by the greater recognition of larger case clusters and super-spreader events (there has been at least one 14-case cluster in a hospital), and also by the possibility that later cases are being recognized more completely than earlier cases, all of which would have the tendency of biasing R0 estimates upwards. I’ll assume that the lower bound R0 (around 2) is probably about right, and also that this is consistent with estimates from SARS coronavirus, which shares substantial genetic similarity with nCoV.
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