A recent paper out of France Estimating the burden of SARS-CoV-2 in France looked at what might happen in France if the current isolation measures, scheduled to ease on May 11, are in fact eased.
A couple of relevant numbers they used were
Infection Fatality Rate (IFR) = 0.53%
R0 prior to lockdown (i.e., normal life) = 3.31
Their IFR value (which is different than the more commonly cited Case Fatality Rate (CFR)) is consistent with a number of other studies. In short, one half of one percent of the people who get infected will die.
Reproduction number R0
The effective reproduction number, R0, has a lot of wiggle room and is dependent on many factors. The section “Variations in R0” in the paper Complexity of the Basic Reproduction Number (R0) provides a list of factors that create that wiggle room.
For example, in the paper on France, the reproduction number R dropped from 3.31 down to 0.52 after the lockdown went into place. An R below 1.0 means the disease is dying out.
Herd immunity
To control an infectious disease a society needs to establish herd immunity. That herd immunity can be accomplished through
natural immunity (you are just immune by nature)
naturally acquired immunity (you caught the disease and recovered)
vaccination
With the SARS-CoV-2 virus being novel, there seems to be little to no natural immunity.
With no vaccine available probably until sometime in 2021, it looks like the only way path to immunity right now is to get the disease and recover.
What percentage of the population needs immunity for herd protection to kick-in?
The general calculation is (1 - 1/R0)
Using that equation, the estimates for the percentage of people who must become infected and recover for herd immunity to be effective at stopping this infection for various numbers of R0 are:
R0 = 5.0: 80% of the population
R0 = 3.3: 70% of the population
R0 = 2.0: 50% of the population
R0 = 1.5: 33% of the population
The French paper used the R0 = 3.3 (when no isolation steps are implemented), so 70% of the population needs to be infected or get the vaccination before normal life can continue.
Death toll, R0, IFR, and the population size
Combining R0 (and thus the derived herd immunity level) with the IFR, and the size of a population gives us an estimate of the expected death toll.
Expected death toll = Population_size * (1 - 1/R0) * IFR
For example, using the following figures:
USA population = 330,000,000
IFR = 0.53%
R0 = 3.3
With these numbers the expected death toll in the United States is 1,219,000 — that’s 1.2 million dead.
This number closely reflects the calculator The NY Times created for this March 16 article (adjust the infection rate and fatality rate sliders to get a different estimate using different assumptions):
Screenshot of NY Times article with 70% of population infected and an IFR of 0.5%
Right now the only variable we can control is R0, and we do this through social isolation measures.
Some huge caveats
A major factor determining R0 is a population’s density (again, see this paper for a list of factors effecting R0). In general the higher the population density, the higher the R0. France has a population density of 309 people per square mile, whereas the United States has a population of only 94 people per square mile. I guess you can thank fly-over country for a lot of that lower density in the USA.
And certainly the population density of New York City is different than rural North Dakota,
I suspect the most important number to pay attention to is the effective reproduction number for your region for a given level of social isolation. For example, if a region implements “Isolation Plan 1” they may have one effective R, but if they implement “Isolation Plan 2” they may have a different effective R.
My personal opinions
My personal opinions are:
(1) Let different regions select their own measures of social isolation, but they should do this with their eyes wide open.
Unfortunately I see too many people saying stupid things like “My farts go through my blue jeans, so how can a mask help.” It is like these people revel in their ignorance. That level of ignorance is not eyes wide open.
(2) The United States should be investing serious money (like the Manhattan Project level of investment) for a vaccine. Bill Gates is making a major effort, but the United States Government, perhaps in a coordinated effort with many other countries, should be pouring massive resources at this problem.
Unfortunately, Trump’s effort to defund the World Health Organization in the middle of a pandemic is pretty much the opposite of the leadership that the world needs right now.