What is the percentage of people who need to be immune against COVID-19 in order to achieve herd immunity?

Earlier in the COVID-19 pandemic, the idea of herd immunity was thought to be the golden ticket out of this public health crisis. But with the dangerous Delta variant becoming the dominant strain of SARS-CoV-2 in the U.S., many Americans delaying or refusing COVID-19 vaccination, and people urged to mask up again in public indoor settings, the concept of herd immunity can seem as though it is out of reach.

Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease, limiting further disease spread. For those who are not immune, they are indirectly protected because the ongoing disease spread is small. Learn more from this JAMA Patient Page, “What is Herd Immunity?”

Some epidemiologists have estimated that 70% of the worldwide population would need to be vaccinated against COVID-19 to establish herd immunity against the disease that’s already killed over 620,000 Americans. But, with the Delta variant driving up cases among the unvaccinated as well as causing breakthrough COVID-19 infections, the threshold for herd immunity now seems to be higher.

Two AMA members took the time to discuss what patients need to know about herd immunity to clear up some of that misinformation. They are:

• Peter Hotez, MD, PhD, dean of the National School of Tropical Medicine and professor of pediatrics and molecular and virology and microbiology at Baylor College of Medicine and Texas Children’s Hospital in Houston.
• Thompson W. Liddell, MD, an infectious disease specialist at Hattiesburg Clinic—a member of the AMA Health System Program.

“I’ve stopped using the term,” said Dr. Hotez, noting that some groups, especially those with political agendas, are using unrealistically low herd-immunity estimates to argue that “we don’t have to worry about masks, vaccines or physical distancing.”

Instead, Dr. Hotez has “started saying that these are the levels of vaccination we need to get to in order to start slowing or even halting virus transmission.”

“Herd immunity really requires the whole herd and we're a long way from that,” said Dr. Liddell, noting that barely half of the total U.S. population has been fully vaccinated against COVID-19. “We're seeing progress towards it and people are starting to look at this new wave and say it is time I go get vaccinated—it’s pushing people as they see people get sick and they're hearing about the ICUs filling up again.

“All the concern that we have now—and I hope people do know that we are concerned—it's terrible to have this come back again in a new wave,” he added, noting that “we're dealing with a worse version of the virus than we were before, so hopefully we're going to have some people change their minds on getting the vaccine … and we can get a little control over this.”

“Technically what herd immunity means is, depending on the level of transmissibility of an infectious agent, if enough people become infected and recover—and therefore are partially immune—or if they get vaccinated, it means that that will diminish, reduce or even halt virus transmission in the community,” Dr. Hotez explained.

“Herd immunity is a confusing topic because it’s different for different things,” said Dr. Liddell. “But the general idea of herd immunity is that you have protection in a community.”

“Even though a person may not have immunity for whatever reason—maybe they don't have a strong immune system, they're unable to get the vaccine or children are the obvious example right now—what we do is we surround them,” he said. “We insulate them from the virus by having everybody around them have immunity.”

“The best example is measles. Measles is one of the most transmissible agents,” said Dr. Hotez. “We know it has a reproductive number of between 12 and 18, and what that means is if you have over 90% of the population immune—either because of infection or vaccination—it can halt transmission. This has been more or less the situation for the last two decades in the U.S, until measles transmission resumed in 2019 due to local declines in immunization rates due to antivaccine targeting of parents.

“The way that 90–95% is often theoretically calculated is to use a simple equation: 1 minus 1 over the reproductive numbers, so 1 minus 1 over 12,” he added. (1 divided by 12 equals .083; subtracting that from 1 equals .916 or about 92%),” he added. “It’s kind of held up because once you get vaccination coverage below 90%, then you do start seeing breakthrough measles outbreaks.”

“People want to know what’s the threshold for herd immunity? What’s the percentage of people who have to be vaccinated?” said Dr. Liddell. “We can guess … but we get very different answers because probably the most complicated piece of it is that COVID is changing.

“The Delta variant that we’re dealing with now is more transmissible, which is going to change the percentage that we need for herd immunity,” he added. “It's now harder for us to achieve herd immunity with the Delta variant than it was with earlier COVID.”

“We're fighting a moving target and the longer we let it move, the longer we let it change into something worse, the harder it is.” Dr. Liddell explained.

“The original lineage was thought to have a reproductive number of around two or three, so 1 minus 1 over 3 would be two-thirds, or 60% to 70% immunization or infection to create some kind of herd immunity,” said Dr. Hotez. “The problem, though, is the Delta variant [reproductive number] is much higher. Some say between five and eight. Others say between eight and nine, so we’re looking now at over 85% immunization rates for the entire U.S. population … which means that you’d have to pretty much vaccinate every adult and adolescent to make that happen.”

“That’s starting to happen in Vermont, and you are seeing a decline,” he said. “You’re not seeing the big upswing in transmission like you’re seeing here in the south where vaccination rates are so profoundly low.”

“If the majority of people are safe and have herd immunity, but there are populations where the virus is allowed to continue, the risk overall is lower because there's less virus happening,” Dr. Liddell explained. “So, less virus happening means fewer replications, which means fewer mutations, fewer variants. But it is still happening.”

“And with an infinite amount of time, even if it's happening less, it's still happening to that degree over time, so you're always going to have that risk,” he added.

“Vaccination is important in halting vaccine preventable diseases,” said Dr. Hotez. “The most dramatic example was, from my perspective, Haemophilus influenzae type B—Hib meningitis.

“When I was a house officer in Boston in the eighties, I was admitting a very sick child with bacterial meningitis to my service every couple of weeks,” he added. “And over a period of a couple of years, the disease more or less disappeared because of widespread vaccination.”

That means that “vaccinating large cohorts works. It really can halt transmission,” said Dr. Hotez. “By the time I was an attending physician I would practically teach about Hib meningitis for mostly historic interest.”

“The goal of vaccinating is not really herd immunity. That ultimately is a good goal, but the goal of vaccinating right now with COVID is to protect the people around you and yourself,” said Dr. Liddell. “If we really could vaccinate on a large scale, then yes we would have that barrier and we would be able to have herd immunity and we wouldn’t be as sick from this.”

“If it passes through everybody, then it is going to impact a larger amount of people, but if we can keep it controlled, the amount of people who get sick, the amount of people who die is much less,” he said, adding “whether or not we achieve herd immunity, every time someone gets the vaccine everyone's safer.”

“What we need to do is we need to get 85% of the U.S. population vaccinated—we need all of the adolescents and all of the adults,” said Dr. Hotez, noting that “except for a few areas in the Northeast and maybe Pacific Northwest, it's not happening.

“In fact, the opposite's happening in the South and that's why we're seeing Delta rage through the South right now,” he added. “It’s going to be really problematic to get through a school year safely in many parts of the South unless we can fix these vaccination rates.”

“Especially for little kids, parents are the No. 1 person they’re interacting with,” said Dr. Hotez. That means “parents absolutely should get vaccinated.”

“I have four children. The best way I can keep them safe is as many people as possible around them being vaccinated, washing hands, wearing masks and isolating,” Dr. Liddell explained. “Obviously, if they stay by themselves all the time, that's the safest they're ever going to be, but that's impractical and ridiculous.”

“It makes sense for people who can get vaccinated around them to be vaccinated,” Dr. Liddell said. “So, that’s how I’m going to keep my kids safe. They’ll be safe if I can create a safe environment for them, like we try to do with everything else.”

Discover what doctors wish parents knew about keeping unvaccinated kids safe.

The AMA has developed frequently-asked-questions documents on COVID-19 vaccination covering safety, allocation and distribution, administration and more. There are two FAQs, one designed to answer patients’ questions (PDF), and another to address physicians’ COVID-19 vaccine questions (PDF).

In the months following the emergence of SARS-CoV-2, “herd immunity” was frequently cited as the long-term destination of the COVID-19 pandemic. As vaccination has rolled out, variants have emerged, and as cases surge once again, we are learning more about the nuances of SARS-CoV-2 infection and what short- and long-term immunity to this virus may look like. With this changing perspective, how should we be thinking about herd immunity?

In this Q&A, epidemiologists Gypsyamber D’Souza and David Dowdy explain that herd immunity is still possible for COVID-19, but that we might need to think a bit differently about what that means in this phase of the pandemic.

What do epidemiologists mean when they talk about “herd immunity”?

When most of a population is immune to an infectious disease, this provides indirect protection—also called population immunity, herd immunity, or herd protection—to those who are not immune to the disease.

For example, currently over 90% of all children in the U.S. are vaccinated against measles, mumps, and rubella by their second birthday. This level of vaccination provides protection to the population as a whole—even to those who aren’t vaccinated—by decreasing viral circulation and the chance someone who is unvaccinated will encounter the virus. If a person with measles were to come to the U.S., for example, nine out every 10 people that person could infect would be immune, making it very hard for measles to spread in the population. As a result, even though we still see localized outbreaks of measles in the U.S., those outbreaks generally die down without starting a nationwide epidemic.

The same idea works for any infectious agent, including coronavirus. The hope is that the population can develop a high enough level of immunity to keep spread low. 

The more contagious an infection is, the higher the proportion of the population that needs immunity before infection rates start to decline. But this percentage isn’t a “magic threshold” that we need to cross—and it’s not just dependent on the level of population immunity. Both viral evolution and changes in how people interact with each other can bring this number up or down as well. But even below any “herd immunity threshold,” immunity in the population (for example, from vaccination) can still have a positive effect by reducing the total number of infections that happen.  

Is herd immunity still an achievable goal for COVID-19?

Yes—but “herd immunity to COVID-19” does not mean that we will soon achieve a level of immunity in the population, like what we see with measles, and coronavirus will be “over.” Returning to life as it was before the pandemic, without seeing large coronavirus outbreaks, is unlikely to happen for several years, for a few reasons.

First, it has proven much harder to get people vaccinated against COVID-19 than against measles. As of September 2021, just over half of the US population was fully vaccinated against COVID-19—even though we know that the FDA-approved vaccines are extremely safe and have remained highly effective, even against new variants like the delta variant. 

Second, young children are still not eligible for the vaccine, and new children (who are susceptible to COVID-19) are born every day. So, until we get vaccines that are approved for use in all ages, there is likely to be ongoing transmission of the coronavirus in kids, who will in turn be able to infect adults, especially unvaccinated ones.

Third, while our vaccines against COVID-19 are very effective and dramatically reduce the risk of infection, they do not reduce that risk to zero. People who have gotten vaccinated can still become infected (so-called breakthrough infections), and some people who have had COVID-19 can get it again. This means that we would need an even higher level of vaccination against COVID-19 to achieve herd immunity. 

For all these reasons, it will be very challenging to get to the level of population immunity against COVID-19 that we have seen with measles in the U.S. We should therefore expect to see some level of ongoing coronavirus transmission in our population for many years (if not forever). But as we build immunity to SARS-CoV-2, the risk of severe illness will decrease, and future waves of infection won’t be as disruptive. 

So, how should we be thinking about herd immunity to COVID-19?

Because we can’t eliminate SARS-CoV-2, there is a real risk that it will continue to circulate until the vast majority of the population will be exposed—but vaccinated individuals are less likely to be infected when exposed, and they are largely protected from the worst outcomes of infection. In the U.S., someone who is vaccinated has less than 1/10th the risk of getting seriously ill as someone who is not. 

This level of protection against severe disease makes it possible that—even if we can’t eliminate transmission of the coronavirus soon—we can get to a level of population immunity where COVID’s effects can be manageable. For example, by achieving high levels of vaccination before the summer of 2021, countries like Germany, Spain, and Canada have kept their COVID-related death rates much lower than what we have seen in the U.S.—even though they have seen surges in cases like those seen in the U.S. COVID mortality rates in these countries are similar to the death rates from suicide or motor vehicle crashes in the U.S. Each of these deaths is a tragedy, but this comparison suggests that, if we can increase vaccination levels, we can eventually get COVID-19 to a level where its effects are like those of other major public health concerns in this country. 

Does that mean we can stop wearing masks indoors or avoiding large gatherings?

Unfortunately, not yet. Eventually, we can expect that levels of serious COVID-19 illness will drop to a point where we will be able to ease most restrictions. But as of September 2021, levels of coronavirus transmission—as measured by the risk of serious disease in an unvaccinated person—are higher in some places than they have ever been. When virus transmission levels are high, it makes sense to take measures that will protect both ourselves and those we love, including getting vaccinated, wearing masks indoors, getting tested any time we are exposed to someone with COVID-19, and avoiding large indoor gatherings. 

What is the final destination for herd immunity and COVID-19?

The number of people who are immune to the coronavirus is increasing every day. This includes people getting vaccinated and, unfortunately, a lot of people getting COVID-19. At some point—hopefully soon —enough people in our population will have SARS-CoV-2 immunity. With higher immunity levels, and continuing some of the behavioral changes that reduce transmission (like wearing masks), levels of infection will fall again. 

Deaths from COVID-19 in the U.S. peaked in January 2021, near the beginning of the vaccine rollout, with more than 3,500 deaths per day. During the September 2021 surge, deaths are approaching 2,000 per day. If we can build up enough immunity in our population, we can keep death rates lower than the ~175 deaths per day we were seeing before this surge. At that point, we can gradually move toward a post-pandemic “new normal” where we still take certain precautions, especially during local outbreaks, but where the pandemic doesn’t impact our lives to the extent it does now. 

We are unlikely to eradicate COVID-19 or even to get it to the level of something like measles in the U.S. But we can build up enough immunity in our population to make it a disease that we as a society can live with. We can arrive at this destination soon, if we get enough people vaccinated—and it’s a destination worth working toward.

Gypsyamber D’Souza, PhD ’07, MPH, MS, is a professor and David Dowdy, MD, PhD ’08, ScM ’02, an associate professor in Epidemiology at the Bloomberg School.

NOTE: An earlier version of this article was published on April 16 but was archived on September 13 in light of new information. 

RELATED CONTENT