The Powerful Allure of R0 < 1

I categorize this as “News”, because I don’t know if we will have enough posts on medicine to justify a whole category for Medicine. Because the coronavirus is affecting life so much, it’s well worth writing about currently.

R0 is the average number of people who will contract a contagious disease from one person with that disease. If you want, you can pronounce it “R naught”.


If R0 is less than 1, and stays that way for a while, the disease will disappear. That means that the virus can still be passed around between a few people (for example, if 1 out of 1,000 people forgets to bring a mask into the grocery store), but over time it will run out of targets and we will be done with this whole thing. If R0 is above 1, that means that a few cases produces more cases and eventually most people will get infected. Once you understand the potential of R0<1 it does become tempting to engineer a lockdown.


We understand the spread of the virus better than we did when lockdowns were first discussed in the US in March 2020. It’s not as simple as every infected person giving the virus to two new people. The way the virus spreads is often through “superspreaders” and superspreader events. That’s why so many large gatherings have been cancelled and continue to be cancelled. With this wrinkle in mind, there is still the overall R0 score of whether the virus is increasing or decreasing.

If you fail to stop the initial spread of the virus (R0 > 1) then a great many people can be infected and eventually the spread will slow simply because the virus is running out of hosts. One of the more controversial topics in Spring 2020 was whether “herd immunity” is or is not something we should want to achieve as a society.

One of the absolute best places to get virus news is MarginalRevolution.com. Today, Tyler has written a new post about herd immunity. I’ll provide some quotes. Much more at the link of course.

… these same herd immunity theorists tended to be less pessimistic than many of the mainstream forecasts…

Now, I don’t recall many of those theorists early on making a prediction about a specific number required for the herd immunity threshold to be reached.  Nonetheless, when deaths and hospitalizations collapsed in Sweden, London, and New York at about 20 percent seroprevalence, obviously it seemed that might be the critical level for herd immunity to kick in.

Then things started to go askew in the last few weeks.  First, it seems like a bad second wave came to an already fairly hard hit Madrid.  OK, you could say Madrid was never had 20% seroprevalence to begin with.  And then what appears to be a second wave has started coming to Israel, with rising hospitalizations.  Finally, it is believed that in Britian R equals about 1.7, and that a second wave of cases is on the verge of hitting London and Southeast England.

Tyler Cowen

Even though no one has all the answers, this is the conversation we should be having. In fact, I’m starting to wonder if we should make a Medicine blog post category.

It was just yesterday that the dreaded and long foreseen 200,000 deaths was confirmed.

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