The trio of Yinon Bar-On, Rob Phillips, and Ron Milo, here augmented by Avi Flamholz, have done it again. In the past, they presented us the astounding numbers for the distribution of biomass on this planet, no less. We highlighted this seminal contribution to our understanding of the living world under the title 'How Much Does Biology Weigh?'
Putting their number-lust to work in another paper, the team now provides us with figures of great relevance to the coronavirus pandemic. The numbers are derived from the published literature, but the choosing was done with exquisite care. What do they present us with? In their words: "The discussion is framed around two broad themes: i) the biology of the virus itself; ii) the characteristics of the infection of a single human host."
And further, "We also discuss a number of questions about the virus, and perform 'back-of-the-envelope' calculations to show the insights that can be gained from knowing some key numbers and using quantitative reasoning… It is important to note that much uncertainty remains, and while 'back-of-the-envelope' calculations can improve our intuition through sanity checks, they cannot replace detailed epidemiological analysis."
The paper deals with eight questions head on:
- How long does it take a single infected person to yield one million infected people?
- What is the effect of physical distancing?
- Why was the initial quarantine period two weeks?
- How do N95 masks block SARS-CoV-2?
- How similar is SARS-CoV-2 to the common cold and flu viruses?
- How much is known about the SARS-CoV-2 genome and proteome?
- What can we learn from the mutation rate of the virus?
- How stable and infectious is the virion on surfaces?
Note their great variety in scope. Surely, there is something here for everybody. The terms and concepts employed here are explicitly defined, for example, 'What are the meanings of R0, 'latent period' and 'infectious period?', 'What is the difference between measurements of viral RNA and infectious viruses?'. And, if that were not sufficient, a glossary is also provided. If you are dubious about glossaries, how about this entry: 'Burst size: the number of virions produced from infection of a single cell. More appropriately called 'per-cell viral yield' for non-lytic viruses like SARS-CoV-2'.
A note of caution: "Note that for about 10 out of 45 parameters, the literature values are from other coronaviruses. We await corresponding measurements for SARS-CoV-2." Does this suggest that the authors will grace us with follow-up articles? We can only hope so.
In keeping with the enlightened policy of the journal, eLife, the comments of the three reviewers and the responses by the authors are published at the end of the paper. This seems to have been a particularly useful exercise that helped the paper achieve an even greater tightness.
I believe that we should we mighty grateful to the authors. At a time when we are daily peppered with massive amounts of coronovirological information, being provided with such forthright and digestible analysis should be deeply appreciated. Or maybe we should be conscious of the poser of numbers. As Rob Phillips says: "People are thirsting for quantitative information"… "When you have a numerical grasp on the facts, you can make more precise predictions about how processes unfold. It's complicated and hard, but we've made headway."