What Is Herd Immunity?

What is herd immunity? Imagine if a community or a herd of healthy people is introduced to an infectious disease. If all the population is vulnerable or susceptible to the disease, then you can imagine that the infection can spread through the community quite rapidly. However, if the majority of the people have immunity to the disease, either by having been vaccinated, or if they have gotten sick, recovered, and now have antibodies to the infection, well, then the disease cannot spread very rapidly. In other words, if the majority of the community or the herd has immunity then that will protect the entire community, even the people who are not immunized.

This is herd immunity. There is a resistance to the spread of infection due to most of the population having immunity. One of the great advantages of herd immunity is that, when the majority of people in a community are immunized against infection, it protects even those who either make a choice not to get vaccinations or immunizations, and it also protects those who are unable to get immunizations such as the very elderly, the very young, or immunocompromised patients, such as patients on chemotherapy, or patients with certain autoimmune diseases.

 

Question: Do you believe that immunizations in a community should be required, such as in the school systems or on certain jobs? Please comment down below.

Examples Of Herd Immunity

Here are some examples of herd immunity, and I'll use the United States as an example. Here in the U.S., we have achieved a herd immunity against the disease, polio. Why? Because most people in the U.S. have been immunized against polio. Therefore, we don't typically see polio. It's not able to spread because most people are immune. So even people in the U.S. who have not received a polio vaccination are not likely to get the disease because it does not typically exist in our community or our herd. Another example of herd immunity is MMR, the MMR vaccination, measles, mumps, and rubella. So measles, for example, we have achieved a herd immunity with measles. Most people have been immunized against measles, so we don't tend to see measles as an epidemic. We have, however, had an outbreak of measles in the U.S., there has been a breakdown of our herd immunity.

 

 

How Can Herd Immunity Stop Working?

When more and more people choose not to get immunized, then you no longer have the protection of the majority of people within a population being protected. And so an infection can spread more rapidly, and more people can be vulnerable. In order for herd immunity to exist, the majority of people in the community must be immunized or have gained immunity against a disease.

 

How Do We Achieve Herd Immunity For Specific Diseases?

So how do we actually go about the business of achieving herd immunity in the community for specific diseases? Well, I've given you a general overview, a general definition of herd immunity, but we actually can calculate how to achieve that protection, that herd immunity for specific diseases by using specific formulas. Yes, we have it down to a science and we're able to do it based on a number, R0 (pronounced R Naught). So what in the world is, R0?

R0 refers to, how many people within a community can get sick from one infected person. It does not talk about how deadly an infection is or how sick an infection will make you, it simply refers to how easily one person who's infected can spread the infection to other people. So using measles as an example, measles is highly transmissible, it's very infectious. The R0 for measles is between 12 and 18. So one person who's infected with the measles virus can get 12 to 18 vulnerable people in a population sick with the measles.

 

Image Source: Wikimedia Commons

 

R0 for COVID-19

The R0 for COVID-19 or SARS-CoV-2 is between two and three. What does that mean? One person who has COVID-19 or Coronavirus can make either two or three vulnerable people sick with COVID-19.

Now here's how we use this number, the R0 to calculate herd immunity, or how many people in the population will need to be immunized or have antibodies to an infection in order to achieve herd immunity.

The calculation is R0 minus  1 divided by R0. So, in the case of COVID-19, if we say that the R0 is 2, then in order for us to achieve herd immunity, we take the R0 minus 1 divided by the R0. So, 2 minus 1 divided by 2, which is 1/2, or 50%.

If we take the R0 of 3 for COVID-19, then we say, 3 minus 1 divided by 3, or 2 divided by 3 (2/3, or about 66, 67%). What does that mean? In order for us to achieve a herd immunity where we can stop the epidemic of coronavirus, then 50% to approximately 66% of the population would have to be immune to COVID-19 in order for us to achieve herd immunity.

 

So using the example of the United States, if we estimate our population to be about 300 million, then we would need approximately 150 million to 200 million people to be immune to COVID-19 in order to achieve herd immunity and to stop the rapid spread of COVID-19.

 

How We Mitigate The Rate Of Disease Infections

Now, what does that mean in reality? If 150 million or 200 million people were to become infected with COVID-19 in the United States, especially within a short period of time, that could overwhelm the health system because a percentage of those people, unfortunately, will die, and a great percentage of those people would need hospitalizations, ventilators, oxygen, and that many people getting an infection naturally would most certainly cause a healthcare system to crash.

This is why we try to mitigate infection instead of having everyone get an infection naturally at the same time. If they're going to get it, we want to try to slow the rate of them getting the infection or reach a plateau so that our healthcare system can match the needs. Some of the ways that we are mitigating the infection are based on the CDC and the World Health Organization recommendations. They recommend social distancing, staying at very least, six feet apart from a person to decrease the chances of transmitting the infection.

They're also recommending wearing masks. The masks primarily prevent the wearer from spreading the infection to other people. Be sure to watch my YouTube video on coronavirus and masks, I actually give you an example of how to make a mask, and I give you an actual demonstration.

The CDC also recommends hand washing, using hand sanitizer, and not touching your eyes or your mouth. All of these things are meant to slow the spread of infection. Again, because even though we would love to have herd immunity, if you don't have a vaccination, then you certainly don't want all of those people being ill and needing medical help at the same time. Of course, when there is a safe and efficacious vaccination or immunization against infectious disease, that's a whole different story. So take SARS-CoV-2 or COVID-19, if we have a safe immunization, then we can push for that 150 million to 200 million people to be vaccinated against COVID-19. And that would be a way to achieve herd immunity without overwhelming the healthcare system.

 

What Is Herd Immunity? Conclusion

Bottom line, until there is a safe and efficacious immunization against COVID-19, all members of the community or the herd who have not already had COVID-19 are susceptible.  So the best way to slow down or to mitigate COVID-19 would be to follow the CDC and World Health Organization guidelines. Again, you want to do hand washing, social distancing, wear masks and etc..

As for herd immunity in general, it is really quite a remarkable phenomenon, because, in the United States, we have had herd immunity against so many infectious diseases for so long, such as mumps, rubella, polio, diphtheria, pertussis, we sometimes take for granted, or we don't realize that these illnesses used to kill and devastate our populations.

But now, because of the herd immunity, because most people are immune to these illnesses, we don't have to deal with them face-to-face, and most of us have never even seen these illnesses. But remember, again, if more and more people make the personal choices not to get immunizations against these infectious diseases, then it will compromise herd immunity, and we can see a resurgence in some of these illnesses. So when you are making your decisions about whether to get vaccinations or not, just be sure that you are informed and keep your community, keep your herd and public health in mind.