UF Health researcher explains herd immunity

It’s a term we’re going to hear much more frequently during the final chapter of the COVID-19 pandemic—herd immunity. 

With millions of Americans being vaccinated against COVID-19 and millions more recovered from the disease, many are looking to the future and asking when the coronavirus pandemic will end. But how to define the end is a surprisingly difficult question, and the answer could hinge on the epidemiological concept of herd immunity.

Dr. Cindy A. Prins, a UF Health infectious disease epidemiologist and UF associate professor, took time to explain herd immunity and how the role it could play in the pandemic’s final stage. 

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Question: What is herd immunity?

Answer: Herd immunity means that enough people have immunity to an illness that even if one person gets infected within the group, it’s not going to easily spread to other people because of that immunity. It breaks the chain of transmission. And so the idea is that you’ve got enough protection in the population that people who are not able to get vaccinated, for example, could still be protected because lots of other people are immune and they kind of almost create a wall around that person and block them from getting infected.

Q: How is herd immunity achieved?

A: There are two paths to immunity. One of them is through what we call natural infection. So, getting infected with COVID-19. It’s still not quite clear how long immunity will last through infection. But it is immunity for some period of time. And then there’s also vaccine immunity, which develops after you get vaccinated against the disease. Vaccines are developed to be a longer-lasting immunity. And so it’s a combination of the two that can contribute to herd immunity.

Now obviously, it’s much more preferable to get vaccinated than to actually get COVID-19 because of the potential side effects of the illness versus the relatively minor side effects of a vaccination. The best choice is always to get a safe and highly effective vaccine.

Q: At what point does society reach herd immunity?

A: You’re hearing a lot of different figures because we don’t really know what that number is yet, since COVID-19 is a new illness. We don’t have a really good handle on the R0 (pronounced r-naught). That’s basically the number of people that can get infected by one other person—a measure of how infectious a disease is. And the level of herd immunity depends a bit on that.

You’ll see estimates upwards of about 70 percent of a population needing to reach immunity through either vaccination or infection before herd immunity is achieved. And then you’ll see estimates that go even higher, more toward 90 percent. It’s one of those things that we’re gaining a better understanding of along the way.

Q: Can one community reach herd immunity while others remain under the threshold?

Dr. Cindy Prins, UF Health infectious disease epidemiologist and associate professor

A: You’ve got people coming and going in communities all the time. We’re not insular. We’re not completely walled off from each other. Even if, for example, Gainesville has a high level of immunity, but a neighboring town doesn’t—well, people commute into town, they go different places. People from Gainesville can travel out of town.

So, herd immunity really needs to be at a larger population level. It’s great to have it within a town. But you’re still going to be able to introduce the virus to people who are not immune because we’ve got people coming and going all the time.

Q: Does reaching herd immunity mean we never have to worry about the coronavirus again?

A: Even if we hit true herd immunity, we’ll still see cases here and there. Look at the flu. You can still have bad outcomes for people who get infected, even if we have fairly good control of the disease. And of course, COVID-19 is far more dangerous than the flu.

We will probably have flare ups of COVID-19 in groups of people who are choosing not to get vaccinated. If someone in a largely unvaccinated group gets infected, COVID-19 can spread through that population even if society at large has achieved herd immunity.

It’s also important to point out in all of this that we’re not positive that we can achieve herd immunity. Some scientists believe it might be out of reach given vaccine hesitancy, the rise of variants and other concerns. That would leave those who have not been vaccinated at risk of becoming ill. And the disease will continue to be spread among those who are not immune, and even among those who carry some immunity.

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