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A coronavirus piñata at a vaccine event in Henderson on Feb. 26, 2021. (Jesse Paul, The Colorado Sun)

Even as thousands of Coloradans are still being infected by the coronavirus each day and as vaccinations roll out to younger children, an unsettling realization is sinking in among the state’s health leaders: The exit door to the pandemic isn’t as close as we once thought it was.

Instead, new research is changing how experts think about immunity to COVID-19, causing some to reevaluate past assumptions about protection and whether the aspired-to goal of herd immunity is even possible.

“I think, for now, you should set aside the notion of herd immunity,” said Dr. Jonathan Samet, an epidemiologist and the dean of the Colorado School of Public Health.

When Colorado’s COVID-19 Modeling Group — a team of university researchers from across the state who use mathematical and epidemiological models to forecast the course of the pandemic — last released a report, in mid-September, it estimated that 70% of the state’s population was immune to COVID-19. That estimate took into account the vaccination rate and the vaccines’ effectiveness. But it also included estimates of how many people had previously been infected by the coronavirus and the immunity they would have gained from fighting off the virus.

In some parts of the state, such as the Eastern Plains or in Adams County, the immunity estimate was even higher — 75% or 80%. With both infections and vaccinations continuing apace, it was possible that those places could now be approaching what was once believed to be the threshold for herd immunity — somewhere around 85% to 90% for the virus’s delta variant.

But when the modeling group released a new report late last week, it contained a surprise: Immunity had dropped across Colorado. The new statewide immunity estimate is 62%. And in some counties or regions of the state, only about half of the population is estimated to be immune to COVID-19.

So why are we going backwards? Samet said the changes are the result of the team reevaluating assumptions about how long immunity lasts and how strong it is to begin with.

“Immunity wanes,” he said. “Part of the problem we have now is that we’re far enough into the pandemic that those infected originally probably have substantial waning of acquired immunity. And of course some of those vaccinated early on also have waning immunity. So I think the picture is a little more complicated.”

New insight into immunity for infected people

That the protection provided by vaccines diminishes over time — especially in the face of the delta variant — is now well known. But new research on immune protection for people previously infected with coronavirus also played a role in the modeling team downgrading the state’s protection level. 

A study released late last month by the federal Centers for Disease Control and Prevention found that people who were unvaccinated but had been previously infected by the coronavirus had a more than five-fold greater chance of being hospitalized with COVID-19 than those who were fully vaccinated against the virus but had never been infected. The data came from a network of hospitals across the country and looked at patients who were discharged from the hospital with diagnosis codes for COVID-related symptoms.

“These findings suggest that among hospitalized adults with COVID-19–like illness whose previous infection or vaccination occurred 90–179 days earlier, vaccine-induced immunity was more protective than infection-induced immunity against laboratory-confirmed COVID-19,” the CDC stated in its report.

Among the hospitals contributing data to the study was UCHealth University of Colorado Hospital. Dr. Michelle Barron, an infectious disease specialist at the hospital who is one of the co-authors of the study, said the results were not that surprising.

“A lot of this has to do with the nature of respiratory viruses,” she said. “We don’t get long-term immunity. It is often time-limited. I think most people logically know this. How many times have you had a cold more than once? Having that natural immunity doesn’t prevent you from getting infected again.”

The exterior of the University of Colorado Hospital on the Anschutz Medical Campus in Aurora, photographed on Oct. 18, 2019. (John Ingold, The Colorado Sun)

The new study contradicts one released earlier this year in Israel, where researchers found the opposite — that immunity from infection is stronger than immunity from vaccination. Overall, the debate over natural vs vaccinated immunity continues to rage.

Barron said her study matches with what researchers know from other diseases. Immunity after infection is generally not a consistent thing. Some people have minor infections that result in only a minor immune response. Others have worse infections and develop stronger immunity.

“There’s a lot less consistency,” she said, “and I think that’s what we’re seeing.”

Rethinking herd immunity

This is something of a death blow in Colorado to hopes for coronavirus herd immunity — the stage at which enough people are immune to infection that the virus can’t find a sufficient number of new people to jump into for sustained transmission.

If immunity is constantly waning, then people are constantly cycling back into the queue to be infected. But Barron said the notion that a place like Colorado could ever achieve herd immunity on its own was always a bit of a fantasy.

“I think herd immunity is this magical thing we hope to achieve,” she said.

The problem, apart from waning immunity, is that the state is not a self-contained bubble. High levels of immunity in Colorado wouldn’t prevent new cases from being introduced by travelers from other parts of the country or the world. Some places in Colorado have already experienced this, when high vaccination rates didn’t prevent local spikes in cases likely related to tourism.

Meanwhile, low vaccination rates in some parts of the world could lead to new and even more infectious variants that are better at evading prior immunity.

“When you talk about herd immunity, it can’t be herd immunity in Colorado or herd immunity in the United States,” Barron said. “It has to be global.”

This is not to say that vaccination or prior infection counts for nothing. Both will likely make future coronavirus waves less severe. And Samet said vaccine booster shots will also help against waning immunity.

But, what it does mean is that the coronavirus will be a presence in our lives for years to come. We will eventually say goodbye to the COVID-19 pandemic. And then we will say hello to endemic COVID-19.

“Which means, basically, we’re not getting rid of it,” Samet said. “We’re probably heading to that point.”

John Ingold is a co-founder of The Colorado Sun and a reporter currently specializing in health care coverage. Born and raised in Colorado Springs, John spent 18 years working at The Denver Post. Prior to that, he held internships at...