Herd immunity occurs when the vast majority of people in a geographic region are immune to a disease, eventually stopping its spread. 

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Since the coronavirus pandemic began, the term “herd immunity” has been flung 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. Experts estimate that from 60% to upwards of 90% of a population must have immunity to a disease for it to stop spreading. Most countries affected by COVID-19 have not surpassed the 1% mark, including the US, which currently has the highest number of cases out of all countries

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? 

Dr. 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.”

Dr. 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 H.G. Wells’s famous story, the Martians who travel to Earth all die from Earth-borne pathogens that the Martians’ immune systems couldn’t fight.

Dr. Vinetz, of Yale Medicine, 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,” Dr. 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 goal, Dr. Vinetz says, is to remove the possibility of sustained transmission. To get to that point, the vast majority of people in a given community, city, state or country would need immunity. To do that, those people either need to contract the disease and recover from it, or get a vaccine against the virus. 

While “active immunity” — 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. That leaves us with the only other option: a vaccine.

How vaccines promote herd immunity

The purpose of vaccines, Dr. 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 a microorganism that is similar but weaker. 

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

Dr. Vinetz again points to measles as the best example of that. It’s easily preventable with a highly-effective vaccination, most commonly known as the measles, mumps and rubella, or MMR, 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 to 4 million yearly cases before the vaccine was introduced, which provides strong evidence that vaccines are highly effective at reducing the occurrence of or eliminating diseases within a region.

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. 

“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.”


Vaccines have been successful in creating herd immunity to several infectious diseases.

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Vaccination development and approval requires careful analysis of the risk-to-benefit ratio, Dr. 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, Dr. Orient explains. “Everything is a trade-off,” she says. 

Dr. 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 diseases 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, “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 had immunity — either naturally or by vaccination — from it when it first infected humans. 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 its rounds through a population.

A vaccination for SARS-CoV-2 is in the works, but it will likely be more than a year until the vaccine becomes available to the public. Without a vaccine, much of the US is currently stuck under shelter-in-place and social distancing orders, waiting for it to be developed, or for people to get the disease and recover from it to create herd immunity.

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

  • Scientists don’t know if people can contract the SARS-CoV-2 virus more than once. 
  • No one knows if a positive coronavirus antibody test means you are immune from contracting or spreading the virus.
  • Research on COVID-19 treatment is ongoing, and there is no finite treatment. 
  • Much of the population is at risk for serious complications, which can lead to death. But even young, healthy people may develop complications that lead to fatality, which raises questions about the nature of the virus. 
  • It’s impossible to know how many people have had the virus and did not report their case, either because they were asymptomatic or did not feel sick enough to get tested, or a test wasn’t available.

Dr. 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 Dr. 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 has not reached 1% of the population as of April 16 — you can track the cases in the US total on the CDC website, which is updated continuously. “It doesn’t require Einstein to figure out that we are nowhere close to herd immunity,” Dr. Vinetz says.

When will we have coronavirus herd immunity?

Whether we will see herd immunity to SARS-CoV-2 in the future depends heavily on a number of factors — particularly, Dr. Orient says, whether or not 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), Dr. 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.

It’s all very complicated, is the point. 

Dr. 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 big resurgence of SARS-CoV-2 in the fall of 2020 simply because it likely will not have died down or disappeared by that point. Unfortunately, that means we will likely be waiting through 2020 and into 2021 to reach herd immunity.

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.