Let’s talk about herd immunity!
I’m Dr. Jeff Kingsley and welcome to another edition of Riding in Cars with Researchers. We’re in this COVID-19 pandemic and people are looking at how do we get over this? How do we get out of this?
When you’re looking at how dangerous a virus is, one of the things you look at is called R0. It’s an R with a zero after it and pronounced R nought. R0 a measurement of, on average, how many other people get infected from one infected person. If the R0 is two, on average, an infected individual infects two more people. And then those two people, on average, will infect two more people each, and those people will infect two more people each. And so you can see that it is spreading the virus through a community.
If the R0 is one, every one individual tends to infect one individual. The spread in the community is not going to get worse. And if the R0 is less than one, then the infection will fizzle out in a community. Every one person infects fewer than one person, and so the infection will just go away on its own.
Now, there are things that drive that number up and things that drive that number down. How easy is it to catch the virus? The more infectious a virus is, that’s going to drive that number up. If asymptomatic people can spread the virus, that’s going to drive the number up because asymptomatic people feel fine, they go to parties, they spread the virus. Infected people that feel like crap tend to stay at home. What drives the number down? Physical distancing, vaccines, and people who are already immune.
So let’s think about this. If you already got the virus and now you have antibodies, well, you can’t catch it. If the majority of a given community has already gotten the virus and has antibodies, they can’t catch it. Well then by default, if one infected person is in that community, there’s nobody else to infect. And so the R0 goes down. If I know that the R0 is 2 – every one person tends to infect two more people – but I immunize a hundred people and I put a hundred immune people in a room together, can I put one infected person in the room? Not that I would do this, but there’d be nobody to infect. And so in my scenario, the R0 in that room would be zero. On average, one infected person infects zero people, because there’s nobody to infect.
The concept of herd immunity that’s moving around in the press today is this concept of just let the virus infect young, healthy people (protect the elderly, people with Type2 diabetes, people who have obesity, people who have other conditions that increase the likelihood that the virus would cause them a lot of harm), let it go through the community and, therefore, we build up herd immunity so that we then drive that number below one.
In theory, that would work, and it does work for certain viruses and it doesn’t work for others. Influenza has been around forever. Well, why is influenza still traveling around the world year after year after year? Well, because herd immunity doesn’t work for influenza.
The common cold is a coronavirus. COVID-19 is a coronavirus. There tend to be similarities around coronaviruses. The common cold has been around forever. Why is the common cold still here? If most people in the world have gotten the common cold, then why don’t we have herd immunity for the common cold? There are lots of reasons. One of which is our bodies tend not to maintain antibodies to the common cold. So you get sick, your body makes antibodies, you get over the cold, but your body tends to only retain manufacturing those antibodies for a few months, and then your body stops making those antibodies. And so the next time you come in contact with a sick individual, you can catch a cold again. Herd immunity will never happen through that mechanism for the common cold.
The vaccines that we’re looking at are not only vaccines to create antibodies, but we’re also looking at ways to enhance the immune response. What we’re hoping to find is that immunity through vaccination is maintained much longer than immunity because you got COVID-19. And if we’re able to obtain that longer-lasting immunity, then that would work. But the concept of just let it go through communities has scientific and ethical factors that really go against us. More people would die. More people would have long-lasting negative impacts from becoming infected. So there’s an ethical dilemma to allowing herd immunity as a pandemic response. And then scientifically it doesn’t really look like it would work with this virus. Vaccinations could make the difference in that argument, but native infection scientifically doesn’t look like it would work. So there are some thoughts on herd immunity, if you are reading about it in the news, or want to know more.
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