THREAD. This story in NEJM (although lacking some detail) about a German cluster of 2019-nCoV cases raises further concerns about transmission from cases both before and after symptoms and from asymptomatic or subclinical cases nejm.org/doi/full/10.10…. #nCoV #NARD 1/ 
Everyone is now familiar with the R_0 (if not, see this excellent tweetorial ). The R_0 is estimated from the current spread in a naive population and is meant to show the potential for spread. It's not an intrinsic property of the virus. 2/
The R_0 is determined by the infectiousness (mostly determined by properties of the virus itself, i.e. biology), by contact rates (sociology, demography) and by duration of infectiousness (interaction between virus and our defences). 3/
So, if the 2019-nCoV had emerged in a sparsely populated area, and not in a megacity, the estimated R_0 would probably have been lower. Anyway, combined with a measure of disease severity, such as case-fatality (CFR), we can get a sense of the potential disease burden. 4/
In the abscence of vaccines and drugs, the only ways to reduce the spread (ie. reduce the effective R, the R_E) is hygiene measures and to reduce contacts between infectious people and the others: isolation of cases for known cases, social distancing for unknown cases. 5/
The effectiveness of case isolation depends heavily on what proportion (_theta_) of cases are infected by sources who are asymptomatic (incubating, recovered or asymptomatically infected). It doesn't help much to isolate the sick if many asymptomatics are transmitting. 6/
We thus need more than the R_0 to know whether the #nCoV epidemic can be stopped. We need the theta. Outbreaks of MERS, SARS and Ebola (pre-vaccine) have all been stopped by isolation of cases + hygiene, even if their R_0 differ, while we cannot stop flu epidemics. 7/
It's the theta, stupid! If there is much transmission of the 2019-nCoV from asymptomatics, it's probably unstoppable. So where does the virus fit in this diagram? _We don't know yet._
Read the theory in @ChristoPhraser 2005 classic paper pnas.org/content/101/16…. #nCoV #NARD 8/8
Read the theory in @ChristoPhraser 2005 classic paper pnas.org/content/101/16…. #nCoV #NARD 8/8
See WHO's latest view on this
