Herd immunity is a topic that has come up a lot as a possible way to end the COVID-19 pandemic. And, while numbers have been thrown around in the past about what portion of the population needs to be vaccinated or infected to achieve herd immunity, top public health officials say it’s just unlikely to happen at this point.
In a new article published in the Journal of Infectious Diseases, Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases (NIAID) and other NIAID scientists write that “classical” herd immunity for COVID-19 “almost certainly is an unattainable goal." Dr. Fauci and his co-authors specifically cite “significant obstacles” like “substantial resistance to efforts to control the spread” of SARS-CoV-2, the virus that causes COVID-19, like the public’s resistance to vaccination and mask-wearing.
They also note that “neither infection nor vaccination appears to induce prolonged protection against SARS-CoV-2 in many or most people,” making it tricky to fully prevent COVID-19, even if you’ve been vaccinated or infected with the virus in the past.
But what is herd immunity, exactly, and why is it so tricky to achieve with COVID-19? Here’s what you need to know.
What is herd immunity?
Herd immunity, aka population immunity or community immunity, is what happens when a significant portion of a population develops immunity to an infectious disease, according to the Centers for Disease Control and Prevention (CDC). That can happen through vaccination or a prior infection with a particular illness, the CDC says.
Once herd immunity in a population is achieved, it’s unlikely that an infectious disease will spread from person to person. Herd immunity also offers protection for people who aren’t vaccinated, like newborns, because the disease can’t easily spread in a community, the CDC explains.
Herd immunity is tricky with Omicron, Delta, and other variants
Dr. Fauci and his co-authors pointed out that SARS-CoV-2 keeps developing variants, and that makes it hard to develop herd immunity against it. “This is a mutating virus—we get new subvariants all the time,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “It’s very hard to get sufficient population immunity where that transmission of the virus is completely interrupted.”
Infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security, agrees. “SARS-CoV-2 isn’t as genetically stable as other pathogens, such as measles, to which herd immunity has been achieved,” he says. “The virus comes from a viral family that is known to mutate to be able to reinfect individuals routinely.”
As a result, Dr. Schaffner says, “we cannot do with COVID-19 what we did with measles.”
Can we reach herd immunity against COVID-19 through natural infections?
Natural infections, along with vaccines, can lead to herd immunity in a population, but it’s tricky to achieve with COVID-19, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. “New variants keep arising,” he says.
And, while someone my develop infection-acquired immunity by getting COVID-19, it’s likely they’ll only have immunity to that particular variant, Dr. Russo says. So, if you were infected with Delta, it’s still possible to get infected with Omicron. “Our ability to go ahead and develop a level of immunity in the population where it will eradicate this virus really isn’t something that’s feasible, whether through infection or vaccines,” he says.
Dr. Russo also points out that SARS-CoV-2 also can live in animals, where it can continue to mutate and then spread to people. “That may be problematic as well,” he says. “Even if we’re able to manage infections in humans—which we won’t really be able to—20-plus animals can support replication of the virus. That’s another huge barrier.”
Dr. Fauci on herd immunity
Dr. Fauci has spoken about herd immunity before, and he originally thought it might be an achievable goal with COVID-19.
Dr. Fauci once said that 60% to 70% of the population needed to be vaccinated to achieve herd immunity, but he told The New York Times in December 2020 that the number is likely closer to 90%. “We need to have some humility here,” he said at the time. “We really don’t know what the real number is. I think the real range is somewhere between 70 to 90%.”
Now, he writes in the paper, it’s just unlikely to happen at all. “If vaccine- or infection-induced immunity to SARS-CoV-2 indeed proves to be short-lived, or if escape mutants continue to emerge, viral spread may continue indefinitely, albeit hopefully at a low endemic level,” he and his co-authors said.
What happens next with COVID
COVID-19 isn’t going anywhere, and Dr. Adalja says that it was “always destined” to become a “seasonal community-acquired respiratory coronavirus.”
Dr. Russo says he expects COVID-19 will eventually be similar to the flu. “We will have intermittent bumps and hope that cases will be mostly asymptomatic or mild disease,” he says. “We will manage it and do our best to control it through vaccination.”
Dr. Schaffner says it’s likely that “periodic boosters” will be needed that “will have to be changed and adapted as the virus changes in our population.”
“It might be the case that we’ll have to get an Omicron booster in the fall or winter—that wouldn’t surprise any of us,” he says. “The big challenge is getting the public to accept it.”