Those who are not vaccinated could be protected by those who are immunized: This protection is called Herd Immunity. This concept is discussed in our last text. Once a certain number of people have acquired immunity*, bacteria**, viruses***, or other pathogens**** will struggle to find a host. Therefore, the question is: how many immunized people are necessary to achieve herd immunity?
Scientists use the basic reproduction number (R0) to mathematically determine the threshold for herd immunity [1]. Imagine a person named John being infected and entering a town of vulnerable people. R0 would be the number of people that John would be able to infect as well. An R0 of 1 indicates that John will infect 1 person. Then, that 1 person can infect another one, and so on. An R0 of 2 indicates that John will infect 2 people, and those 2 people will each infect another 2. So, after only two rounds of transmission, the total number of infected people would be 7. This metric is a useful tool for following how rapidly a virus can spread: A more infectious virus will have a higher R0. Then, to combat the increased spread of infection, a higher number of immune individuals is needed to attain herd immunity.
How many people should be immune to the SARS-CoV-2 virus in order to achieve herd immunity? If the R0 was around 3, then approximately 67% of the population would need to be immune [1]. In a population of 100 people, the number of new COVID-19 cases will start to decrease once 67 people are immune. This can be achieved if the 67 people had previous contact with the virus (natural infection) or if they were vaccinated. However, this is just a rough estimate because the exact number will change based on variables in each population. Kwok and other colleagues (2020) estimated the R0 for 32 countries, based on data up to March 13th, 2020. Last year, some examples of the herd immunity thresholds were 85% in Bahrain, 69.6% in the United States, and 56.5% in Canada [2], but they continuously update.
In a population without vaccination and an R0 of 3, around 60 or 70 percent of people would have to become infected to collectively achieve herd immunity against SARS-CoV-2. This is problematic due to the number of people that might die if the infection runs its course. Indeed, the case fatality rate from a COVID-19 infection ranges from 2% to 3% [3]. Even if the percentage of death is low, waiting for herd immunity to be achieved naturally is unacceptable, especially when there is an alternative: Vaccination. Vaccination will increase the number of immunized people, without the risk of death and severe disease.
*Immunity - an established memory and quick response that our bodies have against infection by pathogens or foreign molecules that enter our bodies. When we combat and survive an infection, we normally develop resistance against the organism or molecule that caused the infection. Vaccines train our bodies to develop immunity against pathogens, without the need to be infected by them first.
**Bacteria - microorganisms invisible to the naked eye that consist of a single cell, though they can group together to form colonies or biofilms. Some bacteria are pathogenic and cause disease, like Salmonella enterica, while others are beneficial to our health, like the ones normally found in the guts.
***Virus - it can be considered a very small and basic life form or an aggregate of molecules that is simpler than a cell. They can be found in the environment or inside living organisms. It has to infect a cell and use its machinery to multiply, since it cannot multiply by itself. Viruses consist of nucleic acids (DNA or RNA), a protein capsid that contains the nucleic acids and, sometimes, an external envelope of lipids. The flu, measles, AIDS and Covid-19, for instance, are caused by viruses.
****Pathogen - any microorganism that can cause infection and disease.
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Written by: Nicole
Edited by: María and Natasha
BioDecoded is a volunteer group committed to sharing accurate scientific information. For more information about vaccines and their safety profile, please see previous posts or consult with your personal physician. If you have any questions about this topic, please comment or send them to our email.
References:
Randolph, H. E., & Barreiro, L. B. (2020). Herd Immunity: Understanding COVID-19. Immunity, 52(5), 737–741. https://doi.org/10.1016/j.immuni.2020.04.012
Kwok, K. O., Lai, F., Wei, W. I., Wong, S., & Tang, J. (2020). Herd immunity – estimating the level required to halt the COVID-19 epidemics in affected countries. The Journal of infection, 80(6), e32–e33. https://doi.org/10.1016/j.jinf.2020.03.027
Cao, Y., Hiyoshi, A. & Montgomery, S. COVID-19 case-fatality rate and demographic and socioeconomic influencers: worldwide spatial regression analysis based on country-level data. BMJ Open 10, e043560 (2020). Available at: https://bmjopen.bmj.com/content/10/11/e043560
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