Natural herd immunity can’t stop the COVID-19 pandemic.
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All rights reserved Painting by Ernes Board, DEA Picture Library, Getty Images Read Caption The history of herd immunity can explain why we need an effective vaccine to beat COVID-19.
Above: Edward Jenner (1749-1823) performs his first vaccination against smallpox on James Phipps, a boy of eight, May 14, 1796, oil on canvas.
Painting by Ernes Board, DEA Picture Library, Getty Images A 'herd mentality' can’t stop the COVID-19 pandemic.
Debates over herd immunity and natural infection arise with every outbreak.
Effective vaccination always wins.
10 Minute Read By PUBLISHED October 2, 2020 Early disease fighters, such as Edward Jenner, Louis Pasteur, and William Farr, suspected if enough people were vaccinated, it could eradicate a disease.
At the dawn of the 20th century, veterinarians more interested in livestock than people seized on the idea and coined the term “herd immunity.” By the 1920s, clever studies with hundreds of thousands of mice vaulted the idea into the mainstream, stirring optimism that making a fraction of a population immune could forestall a devastating outbreak.
But even the trailblazers researching herd immunity were mystified by how to deploy it in practice.
And now it is part of the debate as the COVID-19 pandemic continues to flourish around much of the world.
Some wonder if herd immunity created as people are naturally infected with SARS-CoV-2 coronavirus would be enough to restore society to working order.
For evidence, such as New York City, where approximately 20 percent of the residents have been infected and the caseload has been low and steady for months.
But based on simple math, past experiences with outbreaks, and emerging evidence from the ongoing pandemic, this claim is a fantasy.
“If we had reached sufficient herd immunity in New York, you would expect incidents to continue going down, not to be holding steady,” says , an epidemiologist at the Yale School of Public Health who specializes in the mathematical modeling of how diseases spread.
A vaccine with 50-percent efficacy could spare hundreds of thousands from hospitalizations, chronic health issues, and death—but it couldn’t hit the herd immunity threshold on its own even if everyone took it.
The model assumes an and a basic reproduction number of 3.0.
It also assumes that immunity develops after infection, so does not allow for reinfection, and that a vaccine is the sole intervention among the masses.
SOURCE: PHICOR The reality is that most of the world— —remains susceptible to infection by the coronavirus virus, despite the global toll so far.
Banking on natural infection to control the outbreak would lead to months, if not years, of a dismaying cycle in which cases subside and then surge.
Even if such community-mediated protection were established, it would be constantly eroded by the birth of children and the real possibility that immunity in those previously infected would wane.
Only two infectious diseases have ever been eradicated: the human scourge of smallpox and the cattle-borne germ rinderpest.
All other known afflictions—including such Old World pestilences as , , and —have either been managed through human intervention or remain uncontrolled.
“It's very unlikely that we're going to see elimination of COVID-19 altogether from the population simply through the buildup of natural immunity,” says Pitzer.
But if we add a highly effective vaccine on top of that, Pitzer says, “then it is theoretically possible that we could eliminate the virus” or at least control it.
A crucial step will be communicating how good the vaccine needs to be to stop transmission.
While major health agencies, including the and the , say a COVID-19 vaccine should be at least 50-percent effective to be approved, this benchmark would actually be too low to establish protective herd immunity.
“It doesn't mean that a vaccine that's below this certain threshold will not be useful,” says , professor and executive director of Public Health Computational and Operations Research (PHICOR) at the City University of New York School of Public Health.
“But if you want to be in a situation where you don't have to do social distancing and these other things anymore, then the vaccine really needs to be over 80 percent efficacy.” What we mean when we talk about herd immunity Herd immunity’s prominence in fighting epidemics can trace its origins to the 1920s and the University of Manchester in England.
Inside a lab there, about 15,000 mice per year scurried through what looked like moon bases in miniature.
Members of one city would be exposed to lethal bacteria, while those in a separate city would receive doses of a vaccine along with the dangerous germ.
The duo’s findings— —demonstrated that immunity in a portion of a population could slow an outbreak and protect otherwise susceptible individuals.
“They called it ,” says , a professor of communicable disease epidemiology at the London School of Hygiene & Tropical Medicine, who has about the origins of herd immunity.
Topley and Wilson—along with some help from —helped popularize the idea, particularly through that’s still used by students to this day.
Yet when most people discuss herd immunity today, they’re really talking about what’s known as the “herd threshold theorem.” It’s what scientists are referencing when they say 75 percent of the population needs to be immune against COVID-19 to stop disease transmission, and it’s surprisingly simple to calculate.
Say a germ lands in foreign world, where an entire population is susceptible.
He sneezed in four faces, but three of those individuals were immune,” Fine says.
Three out of four is three-quarters, meaning a 75-percent threshold is needed to reach herd immunity.
Different viruses have their own reproduction numbers, so each has its own herd immunity threshold.
The mass vaccination program , but nearly a decade later, the disease remained endemic in much of Africa, Asia, and South America, with tens of thousands of cases still reported each year.
The virus kept finding hideouts—both in rural areas and high-density cities where it could fester—and ultimately threaten disease-free areas given that the vaccine’s immunity only lasted five years.
The tide turned on December 4, 1966, when a missionary in the southeastern Nigerian region of Ogoja radioed Foege to warn of a new possible outbreak.
That’s a huge advantage—especially when an epidemic nears elimination and mass vaccination becomes less cost effective.
By 1971, an epidemiologist named John Fox herd immunity models that would better incorporate heterogeneity, and decades later it is still standard practice for public health researchers.
The practice is similar to how firefighters clear trees, shrubs, and other flammable debris to encircle a raging wildfire, and it explains why health care workers, first responders, and people in hot spots such as jails .
Stopping Pandemics: An exclusive National Geographic event with Dr. Fauci & other experts National Geographic's exclusive conversation with Dr. Anthony Fauci, Washington, D.C. Mayor Muriel Bowser, and experts from National Geographic and ABC News explores why the COVID-19 pandemic is wreaking havoc on communities and the lessons we can learn from health pandemics throughout history, August 13, 2020.
Our future with COVID-19 depends on us On August 14, , a mathematician at Stockholm University in Sweden, and two other scientists released a model in that estimates how social activity might influence the herd immunity threshold.
They started with the valid assumption that millennials and Gen Z mix more than older people, and so will more readily spread the virus.
Britton’s team landed on a herd threshold of 43 percent—much lower than the 60 to 75 percent you get using the classic equation.
“We don't claim that the number from our model applies in reality,” Britton cautions, adding that the model merely shows the degree to which disease-induced immunity can play a role.
“We don't want our paper to have the consequence that people feel relaxed and say, Let's skip restrictions and wait for herd immunity.” Another limitation of heterogeneity modeling, Columbia University’s Shaman says, is that no one really knows how germs spread among people on the street, so it’s difficult to tell what these reduced thresholds mean for real life.
“That completely changes the landscape.” Moreover, recent studies of explosive COVID-19 outbreaks in two different regions suggest the classic herd theorem might be valid.
In Qatar, the herd immunity threshold appears to have been achieved in about 10 working-class communities.
“So 60 percent of the population of Qatar is migrant workers.
But raises questions about whether the city truly achieved community protection—or worse, if .
If the latter, the virus will bounce back even if places reach the herd immunity threshold through natural routes.
This vulnerability would be reinforced by children, who are born without immune defenses and thus are susceptible to catching and spreading the disease.
Another concern for waning immunity would be frequent reinfections that result in severe symptoms, Shaman says.
“This would suggest we're not going to be done with this any time soon, and that prior exposure doesn't lessen your chance of winding up in the hospital,” he says.
Though has been reported worldwide, there’s no evidence yet this is happening on a broad scale.
If society wants to overcome these bleak possibilities and return to life without social distancing and mask wearing, it needs a vaccine that provides a sufficient amount of what’s known as sterilizing immunity, meaning the drug blocks coronavirus transmission.
“I would say the sweet spot is 80 percent,” says CUNY’s Lee, who co-authored about efficacy goals for the COVID-19 vaccine.
“We have to make it clear to everyone that the first vaccine to reach the market may not achieve those efficacy levels,” Lee says.
“It's not that easy to get an efficacy that high for a respiratory virus.” That’s because current guidance says vaccine frontrunners can be approved even if they only provide “functional immunity,” which mainly confers protection against the symptoms of the disease.
The ongoing COVID-19 vaccine trials are not designed to estimate the impact the vaccine candidates would have on transmission, write the authors of the National Academy of Medicine report, adding that we may not learn this impact until well after an FDA approval.