I'm trying to wrap my head around some of the information I'm reading about how to fight Covid-19 and conflicting opinions about how much "social distancing" is required to avert a disaster, or whether social distancing works at all. (my training is in IT and Mathematics, not biology or medicine)

  • Can social distancing (without hard quarantines, tracking as many cases as possible, and instantly isolating any new cases discovered, etc...) actually lead to 𝑟0 < 1 and therefore reduce the number of infections as opposed to just spreading them out to a manageable rate?

  • I think the answer is "No", because of small world network theory: The majority of people respect prefect social distancing, i.e. they maintain zero social contact with anybody outside of their immediate family and the the people they interact with for procuring life essentials. Because the number of people who cannot practice social distancing due to their essential role in society (grocery and pharmacy employees, health care professionals, law enforcement, etc...) will still experience very high transmission rates, a small world phenomenon occurs, where the average chain of transmission is still very short between any two individuals in an impacted area. What is wrong with this line of reasoning?

  • In general, is 𝑟0 < 1 achievable without a cure, a vaccine, herd immunity, or the ability to track infections with very high accuracy and isolating them instantly?
  • Too many questions. SE sites prefer one question only. Commented Mar 22, 2020 at 20:11
  • @GrahamChiu thanks. I reduced the number of questions. Two other SE sights that I am more active on do allow multiple questions, as long as they are connected. Is that different here? Commented Mar 22, 2020 at 22:32
  • We are all subject to question fatigue. There aren't enough answerers here. Commented Mar 22, 2020 at 22:46
  • @GrahamChiu "There aren't enough answerers here" - I imagine that medical professionalz have much more pressing things do than answering SE questions (unlike us dev and machine learning types...) - thank you for all the efforts your community is doing on our behalf right now. Commented Mar 22, 2020 at 23:24
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    You should probably refer to "effective reproduction number R", and not to "basic reproduction number R0" which is the coefficient for the disease (and given society) before control measures are taken; en.wikipedia.org/wiki/Basic_reproduction_number
    – P Marecki
    Commented Mar 23, 2020 at 1:44

2 Answers 2


R0 refers to the number of people an infected person infects others. If you quarantine everyone then R0 drops so that only the ones who get infected are those living with an infected person, which is how most transmission has occurred.

Once they get infected no one else can get infected and transmission stops.

But, you're going to need the army/police to enforce the quarantine to make it work as has been shown in Italy where unsupervised quarantine has failed. China enlisted local community groups to enforce quarantine and they are no longer reporting any new cases from inside China vs newly imported.

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    Thanks. Do you consider "social distancing" and "quarantine" to be the same thing? As far as I can tell, they are not, at least how it is being prescribed in the U.S. Commented Mar 22, 2020 at 22:30
  • Social distancing is maintaining at least 1 m from someone and 2 m from a suspected individual. It fails because many people are not enforcing it. Since the virus is almost all driven by droplets and touch, SD could stop the virus if we didn't have humans involved in the equation. Commented Mar 22, 2020 at 22:47

I will try to answer in a different way because your questions are related only to mathematical terms (growth rate, curve shape). So I tried to guess what you really mean. I am ready to delete the asnwer if it is offtopic for you. Few statements for start:

  • in theory - social distancing can make the covid-19 extinct
  • however, in a real-life (especially EU and US), the best what we have seen so far is the flattening of the curve (slowing down the growth) - we do not really have much data so far to conclude anything.

This directly leads to:

  • It seems that herd immunity is the only solution that seems to be reachable for EU and US countries (correct me if I am wrong).

  • Countries are trying to flat the curve because medical resources are limited

  • Thus the ability to flatten the curve can make a significant difference in the resulting mortality rate


  • we are not really able to test/guess how many people are really infected
  • we do not really track how many people are in serious state (in every country in the same way)
  • we choose to model the spread as a exponential growth, but it is just our approximation and it is of course not really correct (later it will be more like logistic growth, but of course it is also not really correct)
  • the RO is just a parameter of the exponential growth, and it greatly differs from situation to situation
  • only "real" way how to estimated the RO and mortality rate is to wait till the end and count the dead and the survivors

To conclude:

In theory social distancing can fully solve the problem in few weeks. In reallity, we expect to suffer for many months/years.

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