Critical Uncertainties About the Wuhan Coronavirus

Since the end of December, we have seen an expanding outbreak of a new variant of the coronavirus in China, spreading from its epicenter in Wuhan.

There are two critical uncertainties to resolve with more evidence: (1) the transmissibility of the Wuhan strain, which so far appears to be high, and (2) the pathogenicity (CFR), which at this point still appears to be relatively low. And when you hear an estimated CFR, always remember to check the denominator on which it is based (lab confirmed or just symptomatic cases).

When it comes to contagious viral diseases, there is usually a tradeoff between their transmissibility (how easily they spread) and their pathogenicity (how many people who become infected die). Viruses that quickly kill their infected hosts effectively limit their own spread.

The number of infected people who die is measured by the "Case Fatality Rate." However, this is a noisy estimate, because the denominator can be based on lab confirmed cases (which increases CFR) or just symptomatic cases (which lowers estimated CFR). Early estimates (based on very noisy reporting) have reported a preliminary CFR for the Wuhan strain of around 2%. However, this will likely change as more evidence becomes available.

To put Wuhan in perspective, the CFRs for Ebola and highly pathogenic H5N1 influenza are >60%. The 1918 pandemic flu was estimated at 10% to 20% (this strain was also relatively transmissible which is why it killed so many). The 2009 H1N1 "swine" flu CFR was estimated at 5% to 9%. By comparison, typical seasonal influenza has a CFR of one tenth of one percent or less (0.1%).

For other coronaviruses, SARS' CFR was estimated to be around 10%, while MERS' was 35%.

Transimissibility is measured using the “Basic Reproduction Number” (known as “R0” or “R-naught”), which is the number of people who will become infected by contact with one contagious person. If R is less than one (e.g., because of a high CFR), an epidemic will quickly “burn itself out”. In contrast, when R is greater than 1, a virus will spread exponentially.
Initial estimates of R for the Wuhan Novel Coronavirus are very noisy at this point. The World Health Organization has published a range of 1.4 to 2.5

For comparison, here are some historic estimated Basic Reproduction Numbers:
  • 1918 Spanish Flu = 2.3 to 3.4 (95% confidence interval)
  • SARS Coronavirus = 1.9
  • 1968 Flu = 1.80
  • 2009 Swine Flu = 1.46
  • Seasonal Influenza = 1.28
  • MERS Coronavirus = <1.0
  • Highly Pathogenic H5N1 Influenza = .90
  • Ebola = .70
The most concerning finding about the Wuhan Coronavirus are claims that it may be capable of infecting other people before a patient becomes symptomatic (i.e., shows signs that he/she has contracted the virus).

An article in the Lancet (“Nowcasting and Forecasting the Potential Domestic and International Spread of the 2019-nCoV Outbreak Originating in Wuhan, China”) found that, “Independent self-sustaining outbreaks in major cities globally could become inevitable because of substantial exportation of presymptomatic cases & the absence of large-scale public health interventions."

If it is supported by subsequent research, this initial finding will almost certainly lead to the imposition of more travel bans and quarantine measures in an attempt to limit transmission of the virus.

To end this post with a bit of good news, a very recent analysis has concluded that, because the number of new coronavirus cases in China is growing more slowly than the exponential rate implied by its Basic Reproduction Number, quarantine, travel bans, and "self-isolation" measures appear to be having a positive impact (

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