In an October 19, 2020 JAMA article, Herd Immunity and Implications for SARS-CoV-2 Control by Saad et al is an excellent and succinct explanation of herd immunity.
We all want society to return to normal. Can we do that through herd immunity? This article gives the answer. I summarize it here.
What is immunity? Having the antibodies that protect you from the virus.
How do you get these antibodies? Either by catching the virus and growing them yourself as part of the defense process or by getting them from a vaccine.
What is herd immunity? It is when enough people have immunity that exposure to the virus of people who don’t yet is infrequent enough that the virus dies out.
What percentage of the population must have the antibody to reach herd immunity? For Covid-19, 60%. This number is different for each communicable disease.
What percentage has it today? The article posits 10% but admits that this is an assumption.
How long will it take to get from 10% to 60%? The article doesn’t say, but my guess is 10 years if we have no vaccine, sooner if we do.
How much quicker? It depends on how many people are vaccinated and how soon. The USA has a population of 330M. Assuming that 33M (the article estimates 10% of the population) have grown the antibody themselves, 330M × 0.6 – 33M = 165M would need to be vaccinated to get us to herd immunity. To deliver that number of vaccinations will demand a massive manufacturing and distribution infrastructure. Even so, we must do it.
There is an additional factor: the duration of the immunity. The article guesses 2 to 3 years but admits that this is not yet known. Whatever this number, it drives the percentage of population requiring immunity substantially higher.
Conclusion? “…infection-induced herd immunity is not realistic at this point to control the pandemic.” We’ve got to have that vaccine.