Excellent article, Markham! Thanks so much for writing this. This is a concept that has been poorly explained by the healthcare leaders in our country and many remain confused on the basics of how coronaviruses work. A couple of additional comments :
You mention eradication above. Natural herd immunity will never eradicate SARS-CoV-2. Only a vaccine could potentially eradicate the novel coronavirus and the odds of that are extremely low for a respiratory virus. The only noteworthy human virus to be eradicated by a vaccine is smallpox. Polio was close but not fully eradicated. This is not an anti-vax comment. I am 100% pro-vaccine and hope that one is developed for SARS-CoV-2 soon. Even if a safe and effective vaccine is produced, however, it is more likely to help bring about control of SARS-CoV-2 as it becomes endemic rather than eradicate it. This is also what natural herd immunity will lead to — the virus becoming endemic, just like the other 4 endemic coronaviruses and other respiratory viruses like human metapneumovirus and parainfluenza.
One other point you didn’t mention is that antibodies are not the only measure of immunity. Many individuals have immunity to SARS-CoV-2 despite having no detectable antibodies. The recent patient found to be ‘reinfected’ from Hong Kong is an excellent example. What should have been the headline news is the fact that the patient had mild symptoms with the first infection, did not develop a significant initial antibody response, and still managed to be entirely asymptomatic upon repeat infection. This is most likely because of t-cells. Dana Smith wrote a great story on this in Elemental last month. She wrote, “Up to 50% of people have memory T cells that respond to SARS-CoV-2, even if they’ve never had the virus.” This is another reason the threshold for herd immunity may be much lower than some previously thought. Up to half of individuals may not even be susceptible to the virus. Her entire story is here:
There’s Good News About Your Immune System and the Coronavirus
When antibody levels go down, T cells have your back
I have taken care of many hospitalized patients with coronaviruses prior to the novel coronavirus pandemic as well as many with SARS-CoV-2 now during the pandemic. The concept of natural herd immunity is not an excuse to stop wearing masks or protect others especially vulnerable people. However, as you suggest, Markham, natural herd immunity is occurring before our eyes. For those who would like to hear more about the concept of natural herd immunity here is a story I wrote 2 months ago. I believe its premise still holds true today.