Herd immunity is when enough members of a population (a herd, so to speak), are immune from a disease, so that a person with the disease infects fewer than one other person.  Once there is herd immunity, outbreaks of the disease taper off rather than grow exponentially.

There are three ways for a person to gain immunity from a particular disease.  They can be born not suspectable to the disease, they can receive an effective vaccination for the disease, or they can get the disease and recover, in the case of many diseases.

We can do nothing about whether or not someone is born with immunity for a particular disease.  In the current situation there is no vaccine yet for the Wuhan coronavirus.  So, the only way one an acquire immunity is to get the disease and recover from it.

Until a vaccine is developed, the only way to obtain herd immunity from the coronavirus is for enough people to have gotten the Wuhan virus and to have recovered from it.  One question that immediately arises is how many people need to get it and recover for herd immunity to take hold.  The answer depends on the rate that of transmission of the disease or the R₀.  I have seen estimates of the transmission rate ranging from 1.4 to 3.5.  At a transmission rate of 1.4, herd immunity requires a little less than 30% of the population to be immune.  If the transmission rate is 2.0, then herd immunity requires 50% of the population to be immune.  A transmission rate of 3.5 requires a little over 71% of the population to be immune. 

Keeping in mind there is no vaccine today and there is not likely to be one for months, the only way to get herd immunity is for something like 30% to 50% or higher of the population to get the Wuhan virus and recover. 

An implication of this is that while social distancing, hand washing and avoiding touching one’s face are excellent policies to avoid getting the Wuhan virus yourself, they delay herd immunity for the population as a whole.  This creates a free rider situation that is the opposite of vaccination.  When vaccination is available and you don’t get one, you are free riding off of the rest of the population who did get vaccinated or contributed to herd immunity by getting the disease and recovering.  With no vaccination available, if you avoid getting and recovering from the disease, you are not contributing to herd immunity and free riding on those who do contribute to it by getting the Wuhan virus and recovering.

Of course, who wants to come down with this dreaded disease?  But this is the point where it is time to amend the statement about not being susceptible to a disease.  In the case of the Wuhan virus, while we don’t know if anyone is naturally immune to it and still cannot really do anything about whether they are or not, we do know that many people under the age of 65 have this disease with a few mild, if any, symptoms.  About 84% of the U.S. population is under age 65. 

Clearly if people come into random contact with each other and the spread rate is 6 or under per infected person implying herd immunity at 83% immunity for the population, then perhaps the spring breakers in Florida had it right after all.  Maybe the young and the healthy should have stayed on those beaches infecting each other, recovering from their mild symptoms and helping build up the herd immunity while not overwhelming the intensive-care units as we waited for a vaccination to be developed.  The non-free riders of the spring break beaches may have had a point.

Now that may sound foolish to people panicked by the press and the Wuhan virus, but so far, the mortality rate from this virus has been very low for people under 60.  Maybe rather than letting epidemiologists with limited data shut down the economy at great cost, the old and others with pre-existing conditions like diabetes and heart problems should have strictly quarantined themselves while the rest of the population built up the herd immunity. 

It would not have been perfect.  The young may interact more with each and the old may also interact more with each other.  But the herd immunity would have been increased at far less sacrifice in the economy, while we waited for vaccine immunity to become possible.

That strategy would still have meant there would be deaths from the Wuhan virus, but for perspective if mortality were evenly distributed across the year, based on the mortality rates there are about 7955 deaths per day in the U.S. Since in early spring we are still coming out of the flu season, early spring daily death totals may well be a bit higher than 7955 per day.