America came this close to herd immunity.
Last May, “we had enough vaccination and natural immunity to have basically almost achieved a population level of immunity,” said Dr. Eric Topol. “We were getting down to fewer than 10,000 cases a day. We were looking good.”
Then the delta variant moved the goal posts.
With the original version of the virus that causes COVID-19, America's current vaccination rate of about 65% would have been enough to stop the spread.
"If we were dealing with the original, we have sufficient vaccination such that the large-scale pandemic would be over in this country," said Dr. Joshua Schiffer, a physician and mathematical modeling expert who studies infectious diseases at the Fred Hutchinson Cancer Research Center in Seattle.
Unfortunately, the now-dominant delta strain is more than twice as contagious and requires more people to be immune through vaccination or previous infection for the virus to stop spreading, say experts.
“Now we need 85 to 90% vaccinated against delta,” said Topol, vice president for research at Scripps Research in La Jolla, California, and a national expert on the use of data in medical research.
It’s not an impossible number. In countries like Portugal, Singapore and the United Arab Emirates, upwards of 80% of the total population are now vaccinated, and cases and deaths are falling.
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That seems unlikely to happen in the United States, where only 55% of the total population is fully vaccinated, and 12% of Americans say are adamantly opposed to it.
Herd immunity is now effectively out of reach, said Stephen Kissler, an infectious disease fellow at the Harvard T.H. Chan School of Public Health.
“I don’t think it’s realistic,” he said.
What is herd immunity?
The concept of herd immunity is simple: When disease sweeps through a herd of animals, the ones that survive become immune. Eventually, enough have what’s known as natural immunity, and the disease has so few animals left to infect that it dies down or evens out.
The concept got a lot of press early in the pandemic when various politicians and even nations suggested that if young, healthy people got mild cases and recovered, there would be enough immunity that the virus wouldn’t circulate anymore and vulnerable people would be protected.
This was before vaccines were available, and the United Kingdom, Sweden, Brazil and the U.S. under the Trump administration advocated the idea to varying degrees.
On one extreme was a group, which included Florida’s now surgeon general, that in October 2020 published the Great Barrington Declaration. It called for the world to end lockdowns and other transmission prevention measures and embrace herd immunity for COVID-19 to protect the vulnerable while allowing economies to thrive.
The idea was quickly denounced. With a death rate at the time of 1%, COVID-19 would have had to kill 3.2 million Americans for enough people to be infected to reach herd immunity.
For a time, the arrival of COVID-19 vaccines changed the calculation. If two-thirds of Americans had gotten immunized in the spring, the virus would have had so few new people to infect that it could have been largely stopped.
Then the delta variant hit.
At the same time, new data began to show natural immunity wasn't as protective as vaccination, and the benefits of shots began to fade after about six months.
More than a third of COVID-19 infections result in zero protective antibodies, said Dr. Mark Rupp, an infectious disease expert at the University of Nebraska Medical Center.
“I wish it weren’t true,” Rupp said. Many of his patients are convinced that having recovered from COVID-19 is all the defense they need.
The good news is that for people who’ve recovered from COVID-19, a single dose of vaccine gives excellent immunity, Topol said.
“You cannot replicate that with any vaccine we have," he said. "It’s pretty extraordinary.”
So far, Rupp isn’t having much success convincing his vaccine-resistant patients to get a shot.
“I've been pleading with folks,” he said.
When will the pandemic end?
With 55% of Americans fully vaccinated and at least 30% recovered from COVID-19 at least once, how is it possible the pandemic can still be surging in so many places?
America is a big country, and even a small number is a lot of people. While it’s hard to pinpoint the number of people not exposed to COVID-19 either through infection or vaccination, experts put it likely at about 15% of the U.S. population. That’s almost 50 million people – plenty to still be getting sick, said Harvard’s Kissler.
It’s also becoming clear that COVID-19 is not “one and done,” said Lauren Ancel Meyers, a professor of statistical and data science and director of the COVID-19 Modeling Consortium at the University of Texas at Austin.
Reinfection and breakthrough cases are changing the landscape of susceptibility as immunity wanes.
On October 1, the seven-day daily COVID-19 deaths in the United States were at 1,479, according to the Centers for Disease Control and Prevention.
“I find it humbling that the leftover percentage, 15%, is still enough to overwhelm our health care system," said Schiffer.
Experts say endemic COVID-19 could make virus 'manageable'
The optimistic expectation, experts say, is that the pandemic will die down, and the virus will become one of the world's many endemic viruses that continue to circulate but cause much less disease and death.
It's predicted to become an infection that still sweeps through the adult population in the winter, sickening some but generally delivering serious illness only to the very old, those with compromised immune systems, and pregnant women who are unvaccinated, said Dr. Gregory Poland, editor-in-chief of the journal Vaccine.
“Once we get to the point where everybody has been exposed or vaccinated and if – and it’s a big if – COVID does what other respiratory illnesses do, it may be a disease that’s manageable,” said Jeffrey Shaman, an epidemiologist at Columbia University.
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Ideally, babies and toddlers would get it multiple times before making it to kindergarten, experts say. For the vast majority, COVID-19 would be mild as it is today for most young children. By the time they begin school, they would have pretty strong immune protection.
The COVID-19 vaccine would become one of the routine immunizations of childhood, probably requiring several doses and possible boosters if new variants appear, experts say.
Much like the flu, COVID-19 in the Northern Hemisphere is expected to be an illness that shows up in the colder months.
If infected, vaccinated adults would generally have mild or even asymptomatic cases. Unvaccinated adults would be at higher risk for severe disease. With age, the immune system becomes less robust, so annual COVID-19 shots would be especially important for those over 65 and the immunocompromised.
COVID-19 also would likely continue to mutate. In some years it would be very mild, in others more severe.
COVID-19 is still evolving
But will this virus follow the typical path of others that we come to live with?
“That’s the trillion-dollar question,” said Columbia’s Shaman.
There are no guarantees with SARS-CoV-2, which can so quickly mutate. The worst-case scenario is that it evolves into something even more dangerous or more contagious than delta.
"All that has to happen is for a new variant with a greater escape from immunity to come along, and we start all over again," Poland said.
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Public health experts have worried for years about a virus with the infectiousness of SARS-CoV-2 and the death rate of Middle East Respiratory Syndrome Coronavirus (MERS-CoV), which is 32% fatal.
Learning to live with COVID-19 means accepting uncertainty and staying ever-vigilant for what might come, said Rustom Antia, a professor of population biology at Emory University.
“Barring a miracle,” added Schiffer, “COVID will be part of our lives for the rest of our lives.”