In the first year of the pandemic, the term “herd immunity” entered the public vocabulary, part of a broader flow of language that moved from the stream of public health into the cultural ocean.

The term, broadly, means that if enough of a population is immune to a virus, then the virus can be crushed, even if not everyone is immune from vaccination or infection. Enough people are immune that the virus has nowhere to go. Small pox has been eradicated. Measles was declared defeated in the United States in 2000, though cases sometimes still arise.

Through the first several months of the pandemic, there was hope, to put it mildly, that population-level immunity could halt SARS-CoV-2, the virus that causes COVID-19, and end the pandemic. Ross Kedl, a professor of immunology and biology at the University of Colorado’s Anschutz School of Medicine, said the concept had worked well against other viruses. The belief was this:

Enough people would get vaccinated, and then, “for all intents and purposes, SARS-CoV-2 would disappear,” Kedl said. “Maybe a case or two here or there, maybe it would smolder, but as long as we got above X percentage (vaccinated), it would be eradicated and eliminated.”

“The original idea of what constitutes herd immunity … those have undergone some very serious revisions over the last two years,” he continued. “Those early days of predictions have been confronted by the mere facts.”

The concept of population-level immunity, Kedl and other experts said, needs to be recalibrated and reframed within the context of what many describe as the coming new normal. Immunity, both from natural infections and from vaccinations, wanes. New variants are likely to emerge, but what they’ll look like — and how they’ll interact with omicron — is unknown. Even if the virus vanished tomorrow, researchers would still be learning about its properties for years to come. 

But vaccines are still remarkably effective at preventing hospitalization and disease, the experts stressed; they just aren’t going to provide a universal shield against infection, just as influenza inoculations don’t. But, like influenza, which mutates even faster than this virus, vaccines can blunt death and hospitalization. They can stabilize the broader effects on our health system and society. They can help control a disease that can’t yet be killed.

“We’re heading in that direction,” Kedl said, “and the question is, what’s the number we’re shooting for?”

When vaccines were first approved and introduced a year ago, 70 percent became the go-to threshold for what share of the population needed to be inoculated to reach broader immunity. It could be achieved, public health officials in Denver and Colorado said, by summer 2021, assuming readily available vaccines and strong uptake.

Much has changed since then. Variants — alpha, delta, now omicron — have emerged, displacing first the initial strain and then each other and morphing the pandemic in their own images. Experts have learned more about waning immunity, both natural and vaccine-provided. More is still being learned and will continue to be learned for months and years to come.

The arrival of variants was quicker than many expected, said Aaron Wendelboe, a professor of biostatistics and epidemiology at the University of Oklahoma’s Hudson College of Public Health. And before the delta wave had fully subsided, omicron arrived and began to rip through Colorado and the United States. Delta in particular surprised him, he said, with its severity and transmissibility.

The variants have become more contagious — freakishly so, in omicron’s case. But that itself isn’t a barrier to herd immunity, Kedl said. The mutations themselves are the problem. Waning immunity also disrupts the idea that the disease can be wiped out entirely.

“You look back, you can see — ‘Yeah, we’re not going to eradicate this thing,'” Kedl said.

He, Wendelboe and Jon Samet, the dean of the Colorado School of Public Health, said they initially believed population-level immunity could be reached. That changed when variants began to radically reshape the pandemic in summer 2021.

“I think we were naive and oversimplifying (the idea of herd immunity) at the start and thinking that herd immunity could be achieved,” Samet said. “Because what we’ve learned, certainly with SARS-CoV-2, is immunity is not permanent.”

“I’ve been trying not to use the word ‘herd immunity,'” Wendelboe said, “because that implies kind of a longer-lasting immunity similar to what we achieved with the measles vaccine or pertussis vaccine. We really don’t talk about herd immunity in terms of influenza, in terms of variants emerging.”

Each variant’s arrival changes the calculation of what vaccination rate threshold is needed to achieve herd immunity, if such a thing is possible. If the magic number for herd immunity was 70%, Wendelboe said, then it’s 90% with omicron, given its rate of spread. A year after the vaccine first arrived in Colorado, just over 70% of residents are fully vaccinated; that rate’s rise has slowed dramatically as time has passed.

He said he expected new variants every six months or so and that new boosters would be needed on roughly the same schedule. The unknowns, of course, are what any new variant will look like. To displace omicron’s place atop the throne, it’ll need to be more contagious, but making any sort of prediction around this virus is difficult, at best. Kedl noted that the natural progression of viruses isn’t to become more deadly, but to become more mild and more transmissible. More mild means the host is more likely to live, giving the virus a better chance to spread and thrive. More transmissible means more hosts to infect.

Researchers “assume” SARS-CoV-2 has a limit for how infectious it can become, he said. Omicron is already “knocking on the door” of measles, considered to be one of the most contagious diseases in the world. But again: Predicting what this virus will bring is like peering into a muddy crystal ball, as Lisa Miller, an epidemiologist with the Colorado School of Public Health, once put it.

The arrival of omicron is one such prediction obscured by the mud of the pandemic. Despite being more mild, it has not thus far led to a great sigh of relief. Mass infections have hampered all areas of public life, from restaurant operations to school reopenings, from the exacerbation of already critical health worker shortages to limitations on whom ambulances can take to the hospital. The disease needs to be checked, experts have repeatedly said, and its effects blunted as much as possible.

But omicron’s record-shattering surge may mean some reprieve, at least for some time, researchers said. Here is where infection clicks with population-level immunity.

“We should enter a period, some months at least, where the population has high level of either vaccine induced or naturally acquired immunity,” Samet said, though he warned that there’s the possibility of a new, “evil” variant emerging. “I guess the other thing that may be helpful is warmer weather, which seems to make a difference.”

Now that omicron appears to have peaked, as public health officials in Colorado this week said it likely has, state experts and researchers are looking at what impact the mass infection will have going forward. Rachel Herlihy, Colorado’s epidemiologist, said she hoped the state would have more information on that soon.

“It’s a legitimate prediction — whether it’s true or not, who knows — but omicron may be much of what we needed to help aid in that transition (toward a post-pandemic society), in part because of its mildness and contagiousness,” Kedl said. “Once you get immunity from omicron, it’s so different from previous strains — the next strain is going to have to pick a different direction to go.”

Omicron immunity “is really protective against delta,” he continued, but not the other way around.

“As long as the strains out there provide cross-protection, then yes, all of this infection will help build that population immunity,” Wendelboe said. But then again: What will the next strain bring? “We could’ve made that argument with delta. All of that infection building toward some level of population immunity — between vaccination and infection, we’ll get there.”

Then omicron came right on the heels of delta in Colorado, shattering infection records and sending positivity rates north of 30%. 

“So,” Wendelboe said.

What comes after that period Samet and Kedl described is, again, murky. Wendelboe said society was driving on the “COVID freeway without an off-ramp in sight.” An off-ramp between surges — involving personal risk calculation and good public health messaging — is important. So, too, is flexibility: A year ago, the classic definition of herd immunity seemed reachable with a 70% vaccination rate. Six months ago, delta was just emerging. Six weeks ago, Colorado was beginning to emerge from delta.

In the interim between those surges, Samet said, vaccinations are critical, as is planning for future waves: more testing infrastructure and supplies, more vaccination, more public health messaging. 

A constant of the pandemic is that understanding has often been, at best, temporary. What comes in the next months is nothing different. What has been consistent over the past year is the success of vaccinations in preventing severe disease and death: State data indicates that the vast majority of those who’ve died in recent months have been unvaccinated. The per-1,000,000 death rate for the unvaccinated was 704 in November, when deaths were spiking. For the vaccinated, it was just under 55. 

If the great unknown moving forward is variants — what they will look like, if they will herald a new, grim turn in the pandemic or a sign of a step away from it — then the way to check them is vaccination, Kedl said. Herd immunity, at least as we should understand it now, isn’t going to kill off SARS-CoV-2. But it can stop those variants from emerging, researchers said, and that in turn can change how we handle the virus altogether.

“The coronavirus tends to spin out in variants based on how much it replicates, so if it’s given a lot of time and space,” he said. “Where the variant has spun out is within the unvaccinated. So one thing herd immunity can do, and our best hope for the future, is the edge to which the population maintains a high level of immunity.”