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Six months into the COVID-19 crisis, the term "herd immunity" has been tossed around as one solution to the devastating virus ravaging countries all over the globe since December 2019.

The term explains the situation in which a high percentage of people in a given community (a city, state or entire country) is immune to a disease, either through vaccination or because those people have been exposed to the virus and their immune systems have built antibodies to protect them from it.

People who tout it as the best defense against SARS-CoV-2, the virus that causes COVID-19, are correct in their thinking -- it's the best way to prevent contagious diseases from circulating throughout a population because it slows down or eliminates the spread of the virus from person to person.

The problem with herd immunity and the novel coronavirus is that the world is nowhere close to having widespread resistance to COVID-19, and is still a long way from developing it -- even after six months of battling out this pandemic. Experts estimate that from 60% to upwards of 90% of a population must have immunity to a disease for it to stop spreading.

Even as of July 2020, as cases have topped 15.3 million worldwide (and 4 million in the US), herd immunity isn't anywhere close. For reference, the US population is about 330 million -- 90% of that is 297 million. One factor standing in the way of reaching that 90% is that large-scale coronavirus research has suggested that people who have recovered from the virus aren't retaining their antibodies, which could mean that herd immunity is farther away than anyone thinks.

Let's explore what herd immunity looks like, what it means for COVID-19 and how the world can get there, explained by Dr. Jane Orient, executive director of the American Association of Physicians and Surgeons; Dr. Joseph Vinetz, a Yale Medicine infectious disease specialist; and the Centers for Disease Control and Prevention.

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What is herd immunity?

Orient tells CNET that herd immunity is what slows and eventually stops outbreaks and epidemics -- this occurs only "when the pathogenic organism runs out of susceptible victims because of isolation, quarantine or immunity of exposed persons."

Orient -- who says she really prefers the term "population immunity" because "we are not livestock" -- emphatically points out that "If it weren't for population immunity … we'd all die like the Martians in The War of the Worlds." In the movie, the Martians who travel to Earth all die from an Earth-borne bacteria that the Martians' immune systems couldn't handle.

Vinetz explains that a good example of herd immunity is the measles.

"The idea is that if we have herd immunity, it means that we reduce the transmission of a disease within a population," Vinetz says. "The best example to think about is measles. If 95% of everyone is vaccinated against measles, that means 95% is immune to measles, which means that the risk of measles spreading from person to person is very low."

The idea, Vinetz concludes, is to remove the possibility of sustained transmission.

While "active immunity," or immunity a person builds in response to actually contracting a disease, is most effective and long-lasting, experts beg people not to intentionally infect themselves with SARS-CoV-2 in an attempt to build immunity.

How vaccines promote herd immunity

The purpose of vaccines, Orient says, is "to expose people to something that induces immunity without making them very sick," explaining that vaccines are either tiny doses of a pathogen, a dead microorganism or one that is similar but weaker.

"You need upwards of 70% of protective immunity in a population to prevent large scale propagation," Vinetz explains, and that often requires immunization, particularly in regard to diseases that have been almost eradicated in the US.

Vinetz again points to measles as the best example. It's easily preventable with the highly effective vaccination, most commonly known as the MMR (measles, mumps and rubella) shot. But, in 2019, the US saw the highest number of measles cases since 1992, and the majority of cases were among people who did not get vaccinated against measles.

The 1,282 measles cases in 2019 still pales in comparison to the estimated 3 million to 4 million yearly cases before the vaccine was introduced, proof that vaccines can eliminate diseases within a region.

"Disease rates are low in the United States today," the CDC explains on its website. "But if we let ourselves become vulnerable by not vaccinating, a case that could touch off an outbreak of some disease that is currently under control is just a plane ride away."

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Only one disease -- variola virus, better known as smallpox -- has been completely eradicated. No cases of naturally occurring smallpox have happened since the declaration of eradication in 1980, and babies no longer need the vaccine for this virus, because the global vaccination program was so successful.

Vaccination development and approval requires careful analysis of the risk-to-benefit ratio, Orient says. "For extremely contagious lethal diseases like smallpox, the risk-to-benefit ratio for vaccines is very favorable," she says. "For milder diseases, not necessarily so."

Vaccines can have adverse effects for some people, too, and immunity wanes over time, so the importance of vaccination depends on a number of factors, including severity and prevalence of the disease, vaccine safety and individual patient factors, Orient explains. "Everything is a trade-off," she reminds us.

Orient urges people not to forget that vaccines, though crucial for some diseases, are not the only way to prevent disease: "Diseases are also prevented by hygiene, sanitation, vector control (e.g. mosquito control), or cure of the sick before disease can be transmitted."

Why we need herd immunity

If there is no herd immunity, people will continue to contract and spread diseases, and, unfortunately, people will continue to die from these diseases. The CDC defines herd immunity, or community immunity, as "a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely."

"Even individuals not vaccinated (such as newborns and those with chronic illnesses)," the CDC continues in its definition of herd immunity, "are offered some protection because the disease has little opportunity to spread within the community."

SARS-CoV-2, the virus that causes COVID-19, is a hauntingly relevant example. This virus is new to humans, so no one has immunity, naturally or by vaccination. Because no one has immunity, and the virus is contagious, it spreads quickly and easily.

What will it take to have coronavirus herd immunity?

There are two ways to produce herd immunity against a disease: develop and administer a safe and effective vaccination, or wait for the disease to make it around through a population.

A vaccination for the SARS-CoV-2 is in the works, but it could be and will likely be, several months to more than a year until the vaccine becomes available to the public. Without a vaccine, shelter-in-place and social distancing orders -- or the polarizing issue of mask mandates -- are the only ways to slow the spread while we wait for a vaccine to be developed.

Vinetz figures that the US is nowhere near the point of herd immunity to the novel coronavirus: "If we have 330 million people in the States and 1% are infected, that's 3.3 million people and that still means that 99% of the US population remains susceptible."

If you take Vinetz's estimation that at least 70% of a population must be immune to a disease to produce herd immunity, that means at least 231 million people in the US must contract and recover from the disease.

The actual number of COVID-19 cases in the US had just barely surpassed 1% of the population as of July 22, 2020 (you can track the cases in the US on the CDC website, which is updated continuously). "It doesn't require Einstein to figure out that we are nowhere close to herd immunity," Vinetz says starkly.

However, too many important factors remain unknown to simply allow the virus to continue its rampage without intervention:

When will we have coronavirus herd immunity?

We've also now seen that striving for herd immunity doesn't work in practice: Sweden, one country that many people looked to as an example because of its leniency during the pandemic, is telling other countries not to follow in its footsteps because the death toll is "unnerving."

Whether we will see herd immunity to SARS-CoV-2 in the future depends heavily on a number of factors -- particularly, Orient says, "whether we allow people to move around."

"If we lock everybody into solitary confinement, they won't get immune, unless and until there is an effective vaccine," she explains. "For some diseases, like the common cold (coronaviruses are one cause) there isn't much herd immunity because the virus changes or the vaccine just doesn't work, like malaria."

When asked about localized herd immunity, or herd immunity within specific regions with high numbers of coronavirus cases (such as New York and California), Vinetz says the concept doesn't really matter if people are allowed to roam around.

"We're such a mobile society," he says, so if stay-at-home orders are relaxed or removed, "if people start getting on buses and planes and trains and visiting other areas," localized herd immunity means nothing.

Vinetz says he expects to see "continual rolling waves of [COVID-19] infection unless we intervene somehow," squashing the idea that there will be a "second wave" of SARS-CoV-2 in the fall of 2020 simply because it likely will not have died down or disappeared by that point.

As of July 2020, Vinetz's predictions are coming true, as coronavirus cases soar once again after lockdown restrictions started easing up earlier in the summer.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.