There’s been a lot of buzz in the news about a term called “herd immunity” and its role during the coronavirus disease 2019 (COVID-19) pandemic.
Herd immunity is the concept that if a sufficient proportion of a population is immune to a germ or virus through vaccination and/or prior illness, it is less likely to spread from person to person.
We talked to UNC Health infectious diseases specialist David A. Wohl, MD to learn more about herd immunity and it implications for COVID-19.
How Does Herd Immunity Work?
Antibodies are proteins that your body’s immune system produces to attack things the body considers to be foreign such as viruses. Depending on the virus, you can become immune—or resistant—to it if you develop antibodies to it.
Once you’ve had a virus or have been vaccinated against one, your body becomes primed to attack that virus even more rigorously the next time you encounter it since the cells that produce antibodies against it have been established and are circulating just in case you are exposed to it again.
If a good portion of people in a community becomes immune to a virus, there’s less opportunity for it to move from one person to another.
“When there’s enough exposure within a group (or herd) through previous exposure or vaccination it can provide protection for others in the herd yet to be exposed. That’s because there is less opportunity for the virus to spread effectively given so many people are protected and, therefore, can’t pass it on,” Dr. Wohl says.
This results in the whole community becoming protected—not just those who are immune. Herd immunity can protect people who cannot be vaccinated and those with compromised immune systems who are most vulnerable to serious illness.
However, viruses are not created equally. For each virus, the proportion of the herd that has to be immune to prevent its spread differs.
“It all has to do with how catchy the virus is,” Dr. Wohl says.
In other words, the more contagious a virus is, the higher the percentage of the population that needs to be immune to it for herd immunity to exist.
“Measles is an example of a really, really catchy virus, and it doesn’t take very many little measles particles to get into your body to make you sick and start a chain of infection,” Dr. Wohl says. “To achieve herd immunity, you need a high number of people to be protected because it’s so easy to catch, and it’s so transmissible.”
For measles, an estimated 90 percent of the population needs to be immune to prevent its spread, Dr. Wohl says. For some other viruses, such as influenza, it’s much lower because they are not spread as easily.
Would Herd Immunity Work for COVID-19?
Some have suggested quickly reaching herd immunity by allowing COVID-19 to spread uncontrolled among the young and healthy population while those at high risk for COVID-19 complications stay sheltered.
This is not a viable option for handling the pandemic, Dr. Wohl says.
First, it’s hard to know yet if herd immunity would work for COVID-19 because it’s a new virus.
“When a novel pathogen, such the SARS-CoV-2 virus (COVID-19), comes along, it’s a different story. We’re vulnerable,” Dr. Wohl says. “It’s new, and we don’t know yet that a vaccination or infection provides protection.”
In fact, the volume of COVID-19 antibodies made drops after the illness ends. Researchers are unsure if that means any protection the antibodies provide against the virus also declines, or if with re-infection, the body would rapidly ramp-up antibody levels.
“Our understanding of this is still evolving,” Dr. Wohl says.
Finally, the cost of letting people get infected can be fatal or have long-lasting side effects. It would endanger those who have underlying conditions that put them at high risk from the virus, which is at least one-third of U.S. citizens. In addition, some young people even without underlying health conditions can get very sick and even die from COVID-19.
“Talk of herd immunity for COVID-19 is, for me, crazy talk. Yes, you can let people get infected, but you’re talking about a disease that can cause death,” Dr. Wohl says. “It’s one thing to look at numbers on a graph. But when you these people in the ICU on a ventilator, they are not numbers on a page or a slide. These are real people who still have rich lives, and to say let’s sacrifice them is cruel and is unethical.”
How Can You Help Stop the Spread of COVID-19?
One way to safely achieve herd immunity to COVID-19 is to have an effective and safe vaccine for the virus that causes it, and researchers, including some at UNC, are hard at work to find an effective one.
People interested in joining well-conducted COVID-19 vaccine studies can register at www.preventcovid.org.
“I understand the public’s hesitancy about new vaccines and the concerns about short-cutting of the research. But, as someone working on this, I can tell you the science is solid and in the next few months we will have rigorous trials that will tell us if these vaccines work and are safe,” Dr. Wohl says.
Until then, the most important thing you can do to protect yourself and help healthcare and frontline workers, as well as those in your community who are at high risk for complications from the virus, is to be sure to wear a mask, remain at least 6 feet from other people, and clean your hands often.
For the latest information on COVID-19, visit the CDC website and the UNC Health COVID-19 Resources page, and follow UNC Health on Twitter, Facebook, Instagram and YouTube.