Herd immunity sounds like something you might hear about in a nature documentary, but if you’ve been following the news this year, you know it’s getting a lot of mainstream attention in relation to COVID-19.
We wanted to better understand herd immunity: What does this concept mean? Why is it so controversial? Could it really be used to end the pandemic? We spoke with two epidemiologists who helped put things into context. As it turns out, herd immunity isn’t a “strategy,” but a scientific measure that helps us understand how infections spread. On its own, it’s not a viable way to combat the spread of COVID-19, and it could put vulnerable people in danger.
What Is Herd Immunity?
Herd immunity is a benchmark used by scientists to determine when a virus stops spreading throughout a community. “Herd immunity is a public health approach to protecting a vulnerable group of the population by ensuring vaccination and immunity to a majority of the populace,” says Saskia Popescu, Ph.D., infection prevention epidemiologist and member of the Federation of American Scientists COVID-19 task force. Normally, this is done through widespread vaccination.
Gypsyamber D’Souza, Ph.D., epidemiologist and professor at Johns Hopkins Bloomberg School of Public Health in Baltimore, explains further: “Herd immunity, or herd protection, is the idea that once a lot of people in a population develop antibodies to an infection, then the other people who haven’t had that infection yet are less likely to get it," she says. “Those people don’t have immunity, but we call it ‘herd protection’ because the other people in the group are less likely to transmit it to them." The unvaccinated or uninfected are not protected, but they’re significantly less likely to be exposed in the first place.
This approach is used to fight viruses like measles and polio, which most Americans have been vaccinated against. “The goal is to vaccinate most people to reduce transmission and protect those who can't get vaccinated and would be at risk for severe disease or death,” Popescu says. For instance, babies under 12 months old can’t get the measles vaccine, so we rely on those who can get vaccinated to do so. This helps keep everyone safe.
Is This a Strategy That Could Work to Combat COVID-19?
On Oct. 4, a group of infectious disease experts signed a statement called the Great Barrington Declaration, advocating for a herd immunity approach they called “Focused Protection” to end the COVID-19 pandemic. In their view, the low-risk members of the community should resume normal life–going to work, attending school, and hosting sport and culture activities. Those at higher risk due to age or pre-existing health conditions should continue to stay home, minimizing all in-person contact with others. This approach has also been promoted by Scott Atlas, M.D., a neuroradiologist at the Hoover Institution in California and a member of the White House Coronavirus Task Force.
In the declaration, the authors argued that ending lockdowns would ultimately help protect the vulnerable by allowing the population to build up herd immunity. “As immunity builds in the population, the risk of infection to all–including the vulnerable–falls,” they wrote. “Our goal should therefore be to minimize mortality and social harm until we reach herd immunity.”
This idea has been met with great controversy by other experts who fear that allowing a return to normal could cause unnecessary deaths. On Oct. 15, a group of scientists published a letter in The Lancet calling the herd immunity approach a “dangerous fallacy unsupported by scientific evidence.” They argued that it could overwhelm healthcare systems and come at great cost to human life.
Here’s the thing: From a public health standpoint, the anti-lockdown approach doesn’t really make sense. “Herd immunity isn’t a strategy,” D’Souza says. “It just describes a situation where people are less likely to get exposed.” She compares it to throwing up your hands and saying, “We give up–let’s get this over with.” Here are five important reasons now is not the time to let the virus just run its natural course:
We’re not anywhere close to reaching herd immunity with COVID-19. Different viruses have different thresholds at which herd immunity is reached, depending on how easily they spread. “The more infectious [a virus] is, the higher the proportion of the population that has to have that protection before you see herd immunity,” D’Souza explains. For measles, that number is 95%; for polio, it’s closer to 80%.
“Studies suggest that for coronavirus, we would see reductions in transmission once 60% or 70% of people in the community had immunity,” D’Souza says. “We are very, very far away from that.” A September study in The Lancet determined that fewer than 10% of Americans have evidence of antibodies to COVID-19 (meaning they were infected at some point, whether they had symptoms or not).
This means that at least half the U.S. population would still need to become infected with COVID-19 for herd immunity to occur. “We do not want to get to those levels by letting the infection run rampant because there would be such a high cost,” says D'Souza. More than 231,000 Americans have died so far, and that’s with the lockdowns in place in many parts of the country. There’s no telling how that number could grow with no precautions at all.
We don’t actually know how long immunity to COVID-19 lasts. This virus has been infecting humans for less than a year, according to official records. At this point, it’s hard to know anything “long term” about COVID-19’s behavior. Popescu notes that these gaps in knowledge make herd immunity a “flippant approach.”
D’Souza echoes this. “We know that there have been cases where people have gotten infected with SARS-COV-2 more than once,” she says. “So, we know that people aren’t always protected once they’re infected.” This will take time for experts to fully understand–so in the meantime, it’s dangerous to assume that widespread infection would lead to widespread immunity.
It would be nearly impossible to insulate the vulnerable from unchecked community spread. We live in an interconnected world. People travel between cities, states, and even continents with ease, meaning that it’s hard to totally isolate one group from another. “We don’t live in isolated communities anymore,” D’Souza explains. “We are constantly traveling back and forth, whether it’s for work, for travel, for pleasure.” Even if one community has stamped out the virus, people who travel in and out contribute to viral spread. “The people who have not been exposed to the infection can get coronavirus if they are exposed,” she says. “And as people travel, that is likely.”
It’s not always clear who will get seriously ill from COVID-19. Generally, the people at highest risk for serious COVID-19 illness are older adults and those with underlying medical conditions like heart disease, kidney disease, COPD, and cancer. But this is by no means a blanket assessment of who will get sickest if infected. “Younger, healthier people can still get seriously ill,” Popescu notes, “and what we're learning about long-haulers means that the [herd immunity] approach would entirely disregard the risk for these people.” Take, for instance, multisystem inflammatory syndrome, a rare but life-threatening complication of COVID-19 that can occur in children. Assuming that young, healthy people will not get sick is essentially just that–an assumption.
We know that masks and social distancing work. There is still much to learn about this virus, but one thing is very clear. Preventative measures like wearing masks, washing hands, and social distancing do slow the spread of COVID-19, and they prevent the healthcare system from becoming overburdened. This way, when someone does need hospital care, they’ll have better access to life-saving interventions like ventilators. Until we get a vaccine, these are our best methods for keeping each other safe–so why would we stop now?
“Even though we can’t go back to life the way it was right now … there are lots of things we can do to keep ourselves happy while still keeping ourselves healthy,” D’Souza notes. We can gather outdoors in small groups, wearing masks and avoiding physical contact. We can spend unlimited time with those in our household or “quarantine bubble.” D’Souza emphasizes the importance of “being smart in the way that we are doing things and using all the available preventive measures to keep ourselves safe.” This pandemic won’t go on forever, but it is our reality right now–so let’s show kindness by doing everything we can to slow the spread.