In a population filled with vulnerable individuals, the introduction of a new virus* results in the rapid spread and infection to multiple people because they are not protected against it. As we have discussed in a previous text, the individuals that have survived infection typically acquire immunity**. This immunity grants protection against the virus because the immune system has already learned how to combat it. When a large portion of the population gains immunity, these people act as a protective “barrier” to those who cannot get vaccinated. This phenomenon is called Herd Immunity. Once herd immunity is achieved, viral spread starts reducing.
For example, in a population of 100 individuals, where no one is immunized, the virus will have plenty of hosts to infect. However, if 80 people acquire immunity to this virus, then the virus would have a much harder time finding a host and spreading. To understand herd immunity and viral spread, it is important to consider the way the virus is transmitted, the period of time in which individuals are infectious, and the length of the immunity after infection, among others.
There are two possible ways to achieve immunity: Either by natural infection or vaccination. During natural infection, it is likely that some of the individuals would die from the virus. On the other hand, vaccination allows them to acquire immunity without the risk of severe disease and death, effectively slowing the transmission of the virus. The vaccine against smallpox is a perfect example of this outcome, discussed in our recent text.
It is important to get vaccinated, not only for yourself but also for others. In society, there will be a few people who will not be able to get the vaccine, like babies, people with compromised immune systems, or people that are allergic to the vaccine and are advised against it by their doctors. These individuals can be “protected” if the rest of the population is vaccinated because it is less likely that they will come across the pathogen*** or an infected individual. In other words, the non-vaccinated individuals are still vulnerable to the virus and they should take precautions. However, if most of the community is made up of resistant individuals, then the virus transmission is slowed: The few vulnerable individuals have less chance of encountering and acquiring the virus and, thus, they become indirectly “protected” [1].
*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.
**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.
***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. We cannot offer any specific health advice. If you have any doubts about getting vaccinated due to previous health conditions, please speak with your healthcare professional or family physician. Your doctor can revise your medical history and advise you on the best path to follow. If you have any general questions about this topic, please send us your questions on the “contact” page or our email.
References:
Randolph, H. E. & Barreiro, L. B. Herd Immunity: Understanding COVID-19. Immunity 52, 737–741 (2020). Available at: https://www.sciencedirect.com/science/article/pii/S1074761320301709
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