Information You Need: What is Herd Immunity, would it work for COVID-19, and what is the cost?

You may have heard about herd immunity in the news recently. Herd immunity is an important public health concept. It refers to the reduced chance of the spread of many diseases, often vaccine preventable diseases like measles and polio. Reaching herd immunity without a vaccine means that a high percentage of the population has to be naturally infected. This could mean a high cost of people sick, hospitalized or dying from the infection.  

What is immunity?

  • If you are immune to a disease or virus it means that your body is able to fight off the germs that cause it so that you never become seriously ill and you can’t pass the virus on to other people. People gain immunity by either (a) being vaccinated or (b) by becoming infected and therefore developing antibodies that help the body resist invading germs that cause a particular disease. 

If someone contracts COVID-19, are they immune?

  • The most recent evidence suggests that antibodies to the virus last at least three months. There is less evidence related to if that prevents people from becoming ill, but we expect that there is at least some protection from these antibodies based on research with other viruses. There is also growing evidence that our bodies T-cells, another part of the immune system, can also provide protection. 

So what is herd immunity?

  • Herd immunity occurs when enough people in the population become immune to a specific disease, which limits the ability for the disease to spread to the small number of people who are not immune. Herd immunity can only occur if a large proportion of the population has been previously infected or vaccinated for a disease. 
  • However, this proportion changes depending on the disease or virus that is spreading. For example, roughly 95% of the population is needed to be vaccinated or infected with measles for herd immunity to occur, while only about 80% of the population needs to be immune to polio to reach herd immunity. This is related to how infectious a virus is. The more infectious it is (i.e. the easier it is to pass between people) the more people need to be immune to stop transmission.

Would herd immunity work for COVID-19 and what are its costs?

  • To reach herd immunity without a COVID-19 vaccine it is estimated that 60-70% of the population would need to be infected with the virus and develop antibodies that cause immunity. 
  • Some rough estimations are outlined below. Note that it is more complex than discussed and depends on the patterns of how people interact with each other and the risk factors in each population (age, health problems, healthcare access and treatment). But this gives a rough idea of the magnitude of potential deaths. 
  • In the United States it is estimated that 0.65% of those who are infected with COVID-19 will die. Therefore, to achieve herd immunity, in the United States 213,330,000 million people would need to be infected to reach the best estimate of 60-70% immune which would be estimated to cause 1,386,645 deaths.
  • By applying the United States estimates to Arizona, we assume that in order to achieve herd immunity, 4,876,930 million Arizonians would need to be infected, which is estimated to cause 31,700 deaths.
  • Globally it is estimated that 0.5-1% of those who are infected with COVID-19 will die. This number is differs in countries around the world due to their age of the population (older more at risk of death, level of health problems, sicker populations more at risk of death)  Therefore, to achieve herd immunity, more than 4.9 billion people worldwide would need to be infected, which could cause an estimated 36,750,000 deaths.
Total PopulationInfection Fatality Rate (Proportion of deaths among all infected individuals)Percentage of people that would need to be infected with COVID-19 to reach herd immunityNumber of people that would need to be infected with COVID-19 to reach herd immunityEstimated number of deaths that would occur to reach herd immunity
Arizona7.279 million (2019)0.65% (United States estimate)67%4,876,93031,700
United States328.2 million (2019)0.65% (crude)67%219,894,0001,429,311
Global7.59 billion (2018)0.5 – 1% (.75%)67%5,085,300,00038,139,750

* These numbers and percentages are only estimates based on evolving COVID-19 data. The table is meant to demonstrate the difficulty and cost of reaching herd immunity without a vaccine. There are limitations to the estimations above because they are based on unadjusted infection fatality rates and do not take into consideration age or other influential factors that may impact the data.

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