The idea that herd immunity would bring an end to the ever-lengthening covid pandemic was probably always a mirage.
In the early days of covid’s global march, when vaccines were still said to be well over a year away and social distancing was the only way to slow transmission, the country needed a goal, a way to conceptualize the pandemic’s end. Herd immunity became that finish line, as experts including Anthony Fauci, now President Joe Biden’s chief medical advisor, communicated a common refrain: Once enough people became immune — either via (catastrophic) mass infection or widespread vaccination — the virus would be starved of new hosts, its rampant spread would stop and normal life would resume.
The details of when and how this seemingly magical threshold would be crossed were fuzzy. Some scientists who study viruses thought only 20-some percent of the population needed to be immune; others estimated it’d be closer to 90. A minority of experts wanted to reach herd immunity as quickly as possible by letting the virus rip through the population rather than wait for vaccines to become available. But no matter the number or method, the concept of herd immunity represented a framework through which public officials and the media (including me) could understand what it would take for the threat to end.
It’s time to update that framework. For this virus, herd immunity was probably never a realistic end goal, said Bill Hanage, an epidemiologist at Harvard University. “Frankly, I wish it had not entered the public consciousness in the way that it has,” he said. “It’s a lot more complicated than people make it out to be.”
That’s because immunity to SARS-CoV-2 is not an all-or-nothing state; instead, it shifts along a continuum of protection that depends, in part, on how many times a person gets infected or vaccinated, which version of the virus or vaccine they get, and how different that is from circulating strains. And protection against infection falls over time.
The rise of BA.5 — the latest in a succession of omicron variants — exemplifies the need to let go of herd immunity. Despite the vast majority of the United States’ population having some form of immunity, the virus rages on, exploiting waning immunity while also evolving around it. After driving waves of infection and reinfection around the world, last week it became the dominant strain in the U.S. and threatens to drive another round of mass infection.
“Permanent herd immunity that makes this virus a thing of the past is not going to happen for the foreseeable future,” said Lauren Ancel Meyers, director of the University of Texas at Austin’s Covid-19 Modeling Consortium. “Immunity is just too impermanent, especially as the virus continues to evolve.”
But that doesn’t mean all is lost. Even if the high bar of herd immunity remains out of reach, populations can still build up protection through vaccination and boosting that lessens the toll of the virus for both individuals and society.
“The development of a sufficient amount of immunity in the population such that we don’t have outbreaks that are devastating to healthcare is a more accurate description of where we are now in some places and where we’re going,” Hanage said. “But that doesn’t trip off the tongue as easily as herd immunity.”
The rise and fall of herd immunity
Herd immunity is a real phenomenon for many pathogens, including measles and polio — both of which have been largely stamped out by widespread vaccination. Once it became clear that SARS-CoV-2 was rapidly spreading around the world and wouldn’t be snuffed out as easily as the original SARS was in 2003, the goal of herd immunity emerged as a hopeful alternative.
But that idea had some major assumptions baked into it, said David Goldblatt, a pediatric immunologist at University College London.
“The first assumption was that once you had an infection, you’d be rendered immune to future infections,” he said. Herd immunity can be achieved for diseases like measles and polio because unvaccinated people tend to get infected only once. Early data hinted that this might be the case for covid, Goldblatt said. The second assumption was that the vaccines, when they came, would confer a similar kind of lasting immunity that prevented infection and transmission.
In this scenario, the immune system’s response is so strong and durable that a wall of population immunity can be built up, one vaccination or infection at a time, until it’s tall enough to keep the virus mostly out.
This dream also rests on the assumption that the virus doesn’t evolve around that growing wall of immunity. The measles and polio viruses have changed very little over time, and the coronavirus’ early trajectory suggested it might evolve more slowly too, at least compared with something like influenza.
Unfortunately “those hopes and expectations were dashed by the behavior of the virus,” Goldblatt said.
Instead of remaining relatively stable, the coronavirus has spawned a series of wildly different variants, with many iterations more capable of subverting immune defenses than the last. The vaccines, while remarkably good at durably reducing severe illness and death, aren’t as good at preventing infection. And whatever transmission-blocking power they induce diminishes over time, especially in the face of omicron, which can spark reinfection within weeks.
With covid, the bricks in that wall of protection erode over time, and they aren’t being replaced fast enough to reach herd immunity. Consequently, many experts suggest it’s time for the public to abandon this way of viewing the end.
“Herd immunity was a useful construct in the beginning of the pandemic for what we thought the end might look like,” said Amber D’Souza, an epidemiologist at Johns Hopkins University. “But because of the immunology of coronavirus and how quickly it has evolved, a more useful construct is to think about what living with covid looks like in the next few years.”
Building up population immunity
Living with covid means accepting the complex and messy weirdness of immunity.
“We often consider immunity to be this sort of monolithic thing,” Hanage said, you either have it or you don’t. “In reality, it’s more complicated than that.”
By now, most of the globe has had some encounter with the virus, either through infection or vaccination. Some are nearly completely immune, having just cleared the virus and enjoying peak antibody levels, which are the front-line defense of the immune system. Antibodies may be waning in those infected months ago, but other parts of their immune systems are still primed to prevent serious illness. Among the vaccinated, two, three or four doses all confer different levels of protection, Hanage said.
“This is a very different environment than the start of 2020, when we were basically a blank page on which the virus could write its own story,” Hanage said. The delta and omicron surges would have been considerably more deadly were it not for this mosaic of varying levels of immunity, he said. “We’re not at the herd immunity level where outbreaks are prevented,” he said, “but the immunity is there, and it’s doing something.” And the more of it, the better.
The virus isn’t going to go away, but continuing to push vaccinations, especially among the most vulnerable, to build up population immunity will help lessen its impact and prevent hospital systems from being pushed to the brink, as some were during last winter’s omicron surge.
Over time, this mosaic of immunity will become more complicated but should grow more protective, Hanage said. How new variants interact with this dynamic landscape is hard to predict, since the virus has thrown curveballs before. But each encounter with covid should become, on average, less severe, on both an individual and population level, Hanage said. That’s no guarantee that some won’t experience more severe reinfections, and the consequences of repeated exposure for long covid remain unclear.
The arrival of omicron-specific boosters this fall will broaden population protection but likely won’t be enough to put herd immunity back on the table, said Walter Orenstein, associate director of the Emory Vaccine Center and a former director of the United States Immunization Program. But ongoing research into new kinds of covid vaccines, including “pan-coronavirus vaccines” that target shared characteristics of many coronaviruses, as well as nasal vaccines that could be better at stopping infection and transmission where it happens, could resurrect the possibility of reaching herd immunity with SARS-CoV-2.
“That would make it easier to get to herd immunity thresholds or community protection thresholds that markedly reduce circulation,” Orenstein said, though convincing enough people to take those new vaccines could be difficult. “I think the likelihood of eradication is zero, forever, but I think we can learn to live with it much like we’ve learned to live with the influenza virus.”
Thanks to Lillian Barkley for copy editing this article.