What is herd immunity? And why should we care? At the present time at least 11 million persons have been infected with COVID-19 worldwide. In the United States, the actual number who have tested positive is a about 3 million as of today (July 3rd). The estimate of actual infected individuals is much higher, probably 10 times higher according to CDC estimates. If that is the case, that represents about 30 million citizens. However, that figure is less than 10% of total population of the United States
Herd immunity is a concept of protection of those individuals that have not seen an agent by all those around them that have been exposed to that agent in the past. This is an important concept for vaccine efficacy. If 60 to 70% of the population is immune to an agent, then it is very difficult for that virus or agent to spread. The virus can not take hold if most people are not getting actively infected. Persons who have been previously infected or vaccinated have an immune system that is primed to kick in and wipe out the invading virus. This will be prevent spread to those who have not had the vaccine, as shown below.
Influenza vaccines have been marketed for years. A recent paper evaluated the coverage of adults by various age groups, demographics, and States (FluVaxView, CDC, October, 25, 2018). At best, in the elderly 66% coverage was achieved in some years. In 2017-2018 total adult coverage was 37.1%. In general, across ten years studied average coverage was 40%.
Given the population of the country a mass vaccination strategy would be required to achieve herd immunity. Two hundred million people would need to be vaccinated to achieve that 70% immunity level. Even if everything goes smoothly, and a safe and efficacious vaccine for COVD-19 can be approved and delivered, I am somewhat pessimistic that adults in our country will accept a new vaccine. The data on influenza vaccine above clearly indicate a general problem. I would hope that is not the case, and we can get our herd protected as soon as possible.