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For almost a year, Americans have been looking forward to herd immunity, when enough people are protected through vaccination or past infection to stop the spread of COVID-19.
Once there, public officials have said, masks won't be necessary and hugging and handshakes – not to mention gyms, bars and indoor dining – can return.
But even as more than half of adult Americans have received at least one dose of vaccine and many others are protected by recent infections, health experts are moving away from the idea of reaching some magic number.
Dr. Anthony Fauci, the nation's top infectious disease doctor, doesn't want to talk about herd immunity anymore.
“Rather than concentrating on an elusive number, let's get as many people vaccinated as quickly as we possibly can,” he said at a White House briefing last week, a sentiment he's since repeated.
What Fauci doesn't explicitly state, but others do, is that with about a quarter of Americans saying they might not want to be immunized, herd immunity is simply not an attainable goal.
“It's theoretically possible but we as a society have rejected that," said Dr. Gregory Poland, director of the Mayo Clinic's Vaccine Research Group. “There is no eradication at this point, it’s off the table. The only thing we can talk about is control.”
After initially aiming for the kind of protection provided by the measles vaccine, officials are now focused on containment similar to the flu: acknowledging there will be regular outbreaks but hoping to limit them as much as possible.
Americans can go through their entire lives without worrying about getting the measles because of a long-lasting effective vaccine given to more than 90% of children. Although small pockets of infection occur when vaccination rates drop, even people who can't get the vaccine or are immunocompromised remain mostly protected.
With COVID-19, where vaccines are effective but won't last a lifetime, vaccine hesitancy makes that kind of widespread protection unlikely, experts say.
That means people who can't get vaccinated or whose immune systems are dampened by medication or disease will remain vulnerable. There will probably always be enough unvaccinated people to allow COVID-19 to spread once it arrives in a community. And even people who are vaccinated won't be 100% protected in the face of such a contagious illness.
But the more people who get their shots, the better.
“We need to pivot the conversation away from thinking of herd immunity as a target we get to or we don’t,” 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. “It’s simple – the more immunity, the better off we’ll all be.”
The immunity divide
Herd immunity has been a moving target as the world has learned more about the newly emerged SARS-CoV-2 virus over the past year.
Last summer, the World Health Organization put the combined infection and vaccination thresholds needed to break the chain of transmission at 60% to 70%. By December, Fauci put the number for the U.S. at 75% to 85%. With the appearance of highly transmissible variants, some have bumped it to 90%.
The unwillingness of some Americans to get vaccinated, however, likely has put the number out of reach.
"What has surprised me most is the incomprehensible rejection of science even among otherwise intelligent people," Poland said. "I’m truly flabbergasted to be watching this on a grand scale."
The split has become political. About 79% of self-identified Democrats say they have been vaccinated or intend to do so soon, compared with 46% of Republicans. About 3 in 10 Republicans say they will definitely not get vaccinated, according to a Kaiser Family Foundation poll.
That means America could end up looking like a patchwork quilt, with areas where COVID-19 infections are low and others where the virus continues to thrive.
“There are going to be places, rural Idaho, for example, where you have very independent-thinking people where there may be continuing spread, because you only get up to 25% of people vaccinated,” said Dr. William Schaffner, a professor and infectious disease expert at the Vanderbilt University School of Medicine in Nashville, Tennessee.
In Tennessee, Schaffner already sees a "striking" divide between the city and the country. "I'm really concerned this virus is going to continue to smolder in rural areas," he said.
In areas of low vaccination, COVID-19 will behave just as it does today.
“People who are unvaccinated are going to be at as much risk of being infected as they ever were,” said William Hanage, an epidemiologist at the Harvard Chan School of Public Health.
The power of vaccination
The dangers of contracting COVID-19 are considerable. Among unvaccinated people who’ve tested positive for COVID-19, about 20% will end up with severe disease, 5% will end up in intensive care and between 1% to 2% will die, according to CDC data.
The unvaccinated will remain susceptible to infection. But so will people already vulnerable – those over 65, who are immunocompromised or have other health problems. Even if they are vaccinated, to stay safe they will have to indefinitely keep up precautions like mask-wearing and social distancing or risk serious disease.
Because of this, public health and infectious disease experts increasingly say herd immunity shouldn't be the focus. Broad vaccination itself can turn COVID-19 from a killer to something much more benign, at least for people who are immunized.
Israel, which at 62% has the world's highest vaccination rate so far, gives a preview of what can happen.
“As soon as vaccination rates hit 50%, you saw cases and deaths just start to plummet,” said Christina Ramirez, a professor of biostatistics at UCLA.
Data from Israel shows that the vaccinated not only are much less likely to get severely ill or die, but if they do get COVID-19, it's almost always a mild case.
“It almost doesn’t matter if the virus is transmitted in the population if it’s not causing serious problems,” said Dr. Timothy Brewer, a professor of epidemiology at the University of California, Los Angeles.
The hope is eventually, vaccination rates rise high enough that the pockets of vulnerability shrink and there is less virus circulating overall.
More people may yet decide to get vaccinated as it becomes clear how much protection it provides, said Ajay Sethi, a professor of population health studies at the University of Wisconsin-Madison.
"I try to be an optimist," he said. "I don’t want to write off rural areas saying they’re forever going to be the communities refusing vaccination. Over time, that will change."
America's COVID-19 future
Another potential wild card is variants of SARS-CoV-2, the virus that causes COVID-19.
For now, the data from Israel is reassuring. Even there, where 80% of the circulating virus was from the more contagious B.1.1.7 variant first detected in the United Kingdom, the Pfizer vaccine was highly effective. The Pfizer, Moderna and Johnson & Johnson vaccines have all been shown to provide some protection against variants.
That could change as new variants emerge, especially if the rest of the world is not able to get sufficient vaccine. Already pharmaceutical companies are working on new vaccines and boosters. Both Pfizer and Moderna said last week people will likely need annual booster doses of COVID-19 vaccine.
That said, experts like Dr. Monica Gandhi, a professor of infectious diseases at the University of California, San Francisco, point to evidence COVID-19 is not out-evolving the vaccines.
"I am a bit surprised by the pessimism being expressed in the press lately about variants and hesitancy not allowing the vaccines to get us through the pandemic. I think they are the solution," Gandhi said.
With the virus still circulating, however, things won't reset to November 2019, before the virus swept the world, UCLA's Brewer said.
"Plexiglas barriers in the supermarket are never going to go away," he said. "But I think we'll get to where there won't be universal mask-wearing."
For many people, COVID-19 may become a background illness like the flu, waxing in the winter and waning in the summer, requiring a yearly or every-other-year booster shot.
“It’s entirely possible that a few years from now, people who are immunized might have a bit of snuffle and it’s actually SARS-CoV-2 but they never know it,” Hanage said.
The future of COVID-19 in the U.S. ultimately will be up to the willingness of Americans to embrace the vaccines, experts say.
“We’re going to be battling pockets of low vaccination for a long time,” said Meyers, of the University of Texas. "COVID-19 is such a stealthy virus – it spreads quickly and silently – it won't start to fade away until the vast majority of the people are immunized."
Contact Elizabeth Weise at firstname.lastname@example.org
This article originally appeared on USA TODAY: Herd immunity in US likely impossible, but vaccines can control COVID