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5 Things to Know About Herd Immunity and the Coronavirus

What it is, how it works, and why a COVID-19 vaccine is considered the key to slowing spread

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En español | With winter fast approaching and cases of COVID-19 continuing to climb in the U.S. and throughout the world, policymakers and health experts are clamoring for ways to gain control over the spread of the coronavirus and to minimize its fallout.

One idea that's made headlines recently is the concept of herd immunity. But what exactly is herd immunity, and how does it work? Here are five things you need to know:

1. Herd immunity is protection for the whole population

Herd immunity is achieved when a significant portion of a population becomes immune to a contagious disease, typically through vaccination, thereby slowing its ability to spread in a community. With nowhere for a disease to go, even those who aren't immune (such as newborns or people who have compromised immune systems) gain protection because you “cut off the channels that allow this thing to spread and take hold,” explains Jeff Levin, an epidemiologist and professor at Baylor University.

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Measles, mumps and polio are examples of diseases to which Americans have developed herd immunity. All three were once common, but with the majority of Americans now vaccinated against them, outbreaks in the U.S. are rare, or in the case of polio, nonexistent.

2. The threshold to reach herd immunity varies

The percentage of people who need to be immune to a disease in order to achieve herd immunity varies, depending on the disease and the physical and social environment, Levin says. Herd immunity against measles, for example, requires about 95 percent of the population to be vaccinated, according to the World Health Organization (WHO). For polio, the threshold is closer to 80 percent.

Experts still aren't sure what the sweet spot for herd immunity to COVID-19 will be, but Ali Mokdad, professor of global health at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, predicts it will be higher than 70 percent. That estimate is based on infection rates in areas of the world where the virus is still spreading, even though a high percentage of those populations have had COVID-19.

3. Herd immunity can be achieved through natural infection

While vaccines are the most common way to achieve herd immunity, they aren't the only route. Immunity can also be acquired through natural infection. When a person is exposed to a virus, the body produces antibodies that help protect against future infections from the same pathogen. So if enough people in a population recover from a disease and mount an immune response, herd immunity can eventually be reached. But there is still more to learn about immune responses developed after a coronavirus infection.

But what's often seen in the absence of a vaccine — and was the case for diseases such as chickenpox before a vaccine was developed — is that even if many adults develop immunity from a past infection, “the disease can still circulate among children and can still infect those with weakened immune systems,” Gypsyamber D'Souza and David W. Dowdy, professors at the Johns Hopkins Bloomberg School of Public Health, write.

A common misconception, Mokdad says, is that herd immunity for COVID-19 has already been reached in areas where infection rates have declined. But “that's not immunity,” he argues. Rather, it's the cyclical nature of the spread of the virus. “That is, you took out some of the key spreaders in that community,” but then “it's going to move to somebody else and somebody will replace them,” he explains.

In the case of COVID-19, studies show only about 10 percent of the global population has been infected with the coronavirus (SARS-CoV-2); U.S. estimates are about the same, meaning many more people would need to get the disease to achieve herd immunity through natural infection. And remember: We don't yet know what that threshold is.

Even in the most optimistic scenario, we'd see about 13 million deaths globally and 1 million deaths in the U.S. from the coronavirus before herd immunity is reached in the absence of a vaccine, IHME predicts. To date, more than 220,000 Americans have succumbed to COVID-19.

4. Striving for vaccine-free herd immunity is problematic

Even still, some argue that obtaining herd immunity through natural infection is a strategy worth considering, especially if those at high risk for severe illness from COVID-19 are shielded from the virus as it tears through the population. However, opponents of the strategy — and there are many — contend that at this stage in the pandemic, allowing the virus to run rampant “could potentially be a disaster,” Levin says.

For starters, the share of the population at risk for severe illness from COVID-19 is significant, and putting measures in place to protect everyone would be difficult. Plus, Levin points out, researchers are still learning about the new virus and the populations most vulnerable to the disease it causes.

So far, it's known that age increases risk for coronavirus complications: Eight out of 10 deaths from COVID-19 have occurred in adults 65 and older, and more than 54 million Americans fall into this age group, according to the latest census data. Chronic health conditions such as heart disease, type 2 diabetes and kidney disease also increase an individual's risk for severe illness from COVID-19. Currently, 6 in 10 U.S. adults have a chronic disease; 4 in 10 have two or more, further elevating that risk. And obesity, which affects more than 40 percent of U.S. adults, has been shown to exacerbate a SARS-CoV-2 infection.

Beyond these high-risk groups, young, healthy people can still get very sick with COVID-19, and some people who recover from it are left with lasting effects of the illness. Abandoning all prevention measures put in place to slow the spread of COVID-19 could “massively overburden” hospitals and create “an enormous surge” in demand for medical care, Levin says.

What's more, experts don't fully understand the immune response people develop after a SARS-CoV-2 infection, including how strong it is and how long it lasts. (A few rare cases of reinfection have already been reported.)

For these reasons, leading health experts have voiced their opposition to the approach.

"Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic. It is scientifically and ethically problematic,” WHO Director-General Tedros Adhanom Ghebreyesus said in a recent media briefing. “Herd immunity is achieved by protecting people from a virus, not by exposing them to it.”

The country's top infectious disease expert, Anthony Fauci, has pointed out the flaws of a vaccine-free herd immune strategy, and Mokdad echoes a similar sentiment. “In my opinion, herd immunity [without a vaccine] basically is a surrender and a failure of the public health system and the medical system by assuming that we give up, that everybody gets infected,” he says. “That's not acceptable.”

5. Until a vaccine, prevention efforts are the best way forward

So then what is the best way to stop the coronavirus from transmitting until a safe and effective vaccine is widely available? “Masks,” Mokdad says. “For the first time in our history as humans, we have the chance to save so many lives by simple, cheap, non-pharmaceutical [masks].”

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Other prevention efforts such as social distancing and avoiding large crowds are also key, Levin adds. “We, as a society, have to take measures that help protect people,” he says.

In early September, Mokdad's IHME projected that nearly 770,000 lives worldwide could be saved by Jan. 1 through proven measures such as mask-wearing and social distancing. The Centers for Disease Control and Prevention (CDC) also recommends frequent hand-washing and routine cleaning and disinfecting as ways to help curb the spread of the virus.

"It requires people to not just think of themselves and to think of what we can do to help be mindful and responsive of other people's needs,” Levin says. “And if we can do that, I think we can hang on until the point that we have some good primary prevention or we have a treatment that works for everybody."

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