Herd immunity is often seen as the end game of the COVID-19 pandemic. This threshold is reached when the virus cannot easily spread because most people have built up immunity through vaccination or natural infection. Then, “the virus just can’t find anyone to infect,” says Monica Gandhi, a professor of medicine and infectious diseases expert at the University of California, San Francisco, and the threat fades.
Experts estimate that we’ll reach herd immunity for COVID-19 when 65 to 80 percent of the population is vaccinated—a number that seemed within reach in the U.S. once highly effective vaccines started being administered. But after peaking at a seven-day average of more than three million doses in early April, the rate declined, and the country is now administering an average of a million doses a day.
Amid a swirl of misinformation, distribution issues, and legitimate hesitancy, as of June 3, only 51 percent of Americans have received at least one shot, and just 41 percent are fully vaccinated.
On June 1, U.S. President Joe Biden announced that anyone in America can get a free round of beer from Anheuser-Busch if they get vaccinated. It’s one of the latest government incentives being offered to help meet his administration’s goal of getting 70 percent of adults at least partially vaccinated by July 4, a key milestone in the nation’s bid to reach herd immunity. Some states are also offering big lottery winnings, free iced coffee, or—in West Virginia—trucks and custom hunting rifles.
Still, other experts say it’s possible the U.S. will never reach herd immunity. In a recent Kaiser Family Foundation survey, 13 percent of U.S. adults said they will “definitely not” get a vaccine. So what happens then? Here’s what scientists have to say about where the U.S. stands now, what that means for the return to life as we once knew it, and why there’s still cause for hope even if we can’t hit the target number for herd immunity.
What is herd immunity?
The concept of herd immunity is fairly simple: Picture a herd of cattle. If one animal in the herd gets infected by a virus and none of the rest are immune, then the virus can spread throughout the whole group, potentially sickening or killing each animal. But if some cows gain immunity, they can block transmission to their neighbors.
Calculating how many people within a population need to develop immunity to protect the rest of the herd depends on a few factors: how quickly a pathogen can spread from person to person; how susceptible the population is to infection; and individual behaviors that can either promote or mitigate transmission, such as whether people adopt masking and social distancing.
Herd immunity occurs when an infected person spreads the virus to an average of less than one other person. In the earliest stages of the pandemic, the average for SARS-CoV-2 transmission was between three and four new cases per infected individual. At that point, scientists believed achieving herd immunity would require 60 to 70 percent of the population to be vaccinated.
But as more virulent variants have emerged that spread more easily, that threshold has risen to 65 to 80 percent, says Saad Omer, an infectious disease epidemiologist and director of the Yale Institute for Global Health. In December, White House advisor Anthony Fauci even suggested herd immunity might require vaccinating up to 90 percent of the population.
But herd immunity is not the same thing as eradication—a fact that experts say has often been misunderstood during the pandemic. Crucially, while herd immunity slows transmission, outbreaks can continue even after a society reaches that threshold, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Think of the measles or diphtheria: Even though the general public doesn’t have to worry about them, local outbreaks still occur. As with seasonal influenza, diseases can also become endemic, circulating in a population at a less virulent level but capable of breaking through when our defenses slip. (Here’s why COVID-19 will likely be with us forever.)
“What herd immunity thresholds get you is more sustainable, more long-term control of the outbreak,” says Omer.
What happens if we don’t reach herd immunity?
Some experts worry, however, that this threshold is slipping out of reach. Even with the authorization of Pfizer’s vaccine for emergency use in adolescents, the New York Times estimates that the country won’t get first doses into 70 percent of its population until October—and that’s only if the vaccination rate doesn’t continue to decline. Meanwhile, large pockets of unvaccinated people remain throughout the country, including children under 12, who are not yet eligible.
Osterholm likens the herd immunity threshold to a weather forecaster reporting on a nationwide average temperature rather than local conditions. Vaccination rates—and the protections that come with them—vary from community to community. While most states in the Northeast have given at least one dose to more than 60 percent of their population, a slew of Southern, states including Mississippi and Louisiana, have reached fewer than 40 percent.
“The nature of coronavirus and other infectious diseases is they find where we’re vulnerable,” says Amber D’Souza, professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. “So wherever the pockets of people are that are not protected, there will be flare-ups in infections. It’s just a question of how long it takes for them to happen.” (The next phase of the U.S. pandemic? Pockets of localized outbreaks.)
Natural infections can help fill in the immunity gap, but Omer says it’s impossible to say for sure how much of the population has natural immunity. It’s also unclear how long natural immunity lasts and to what extent; however, Osterholm says it’s clear that the vaccines offer superior immunity.
If the U.S. doesn’t reach herd immunity, there will be more outbreaks than if the country did cross the threshold. But the situation likely won’t be as dire as it was in January, when hospitalizations and deaths peaked across the nation. Instead, Omer says, there would be periodic smaller surges and loss of life as people head back indoors in the fall and winter.
“That does not mean that we will have to shut down the whole country every time that happens,” Omer says. “But remember if there’s a flood in one month and it’s up to your neck in another month, it’s still pretty bad even if it’s not above your head.”
Osterholm also cautions against a false sense of security in communities with low levels of vaccination—including Southern states—even if their case rates are falling right now. “Why the virus does what it does, why does it come and go, we don’t know,” he says. “We have to be careful not to interpret the absence of cases right now as meaning that we’re protected.”
D’Souza adds that infectious diseases are dynamic: As long as the virus is allowed to circulate anywhere—both in pockets of the U.S. and in countries that don’t yet have wide access to vaccines—it poses a threat to the immunocompromised and others who can’t get vaccines. It also has the potential to give rise to variants that evade the vaccines.
And there’s still much that’s unknown about how long the vaccines protect against the virus and whether booster shots will be needed. Even if the U.S. reaches herd immunity, it could easily be lost.
“One of the challenges we’re going to have with herd immunity is that it’s also not a static situation,” Osterholm says. “Vaccinated today doesn’t mean necessarily vaccinated next year.”
Why there’s reason for hope
But scientists stress that it’s not all doom and gloom.
“I’m concerned about the slowing pace [of vaccinations], but on the other hand there are signs of hope as well,” Omer says, citing the country’s falling rates of cases, hospitalizations, and deaths. “I think that we are seeing the indirect effects of the vaccines kick in.”
Just as herd immunity does not mean a virus has been eradicated, it also is not a hard line that you have to cross to receive any benefits from vaccination. D’Souza argues that herd immunity should be thought of as a gradient rather than a single number: As more people get vaccinated, the entire population is better protected.
“We may never get to 80 percent vaccination, and that could be okay,” she says.
Omer points to the impressive rates of immunization among people who are more vulnerable to death and severe disease. According to the CDC, 75 percent of people age 65 and older are fully vaccinated and nearly 86 percent have received at least one dose. Omer, who was on a committee advising the CDC on how to prioritize vaccination, says this was intentional.
“We all want to see infection rates go down, but we want to see the deaths go down first and more sharply,” he says. Indeed, those numbers have been falling. Nationwide deaths are down from an average of more than 3,500 per day to about 500. New cases have fallen, too, to a seven-day average of fewer than 15,000. “That has given us room to do a lot more stuff socially and physically.”
Gandhi points out that since behavioral restrictions are one way of controlling an epidemic, you need to ease controls somewhat to know whether you’ve actually achieved herd immunity. Some good news on that front comes from Israel, where the country’s case rate had dropped so low by April 19—when just 54 percent of the population had received two doses—that it prompted the government to ease restrictions.
“It was pretty fascinating because as they’ve done that gradual opening, there were no increases in cases,” Gandhi says.
Osterholm says that, while he doesn’t believe true herd immunity is in the cards in the U.S., that doesn’t diminish the importance of getting more people vaccinated. Vaccinating 50 to 70 percent of the population with a vaccine that’s 95 percent effective at preventing COVID-19 will go a long way.
“It’s going to be harder for the virus to find anybody,” he says. “It’ll still find them, but it’ll be slower. And the closer we get to having more people vaccinated, the more protected unvaccinated people will be.”
In the U.S., D’Souza says the coming weeks and months will be critical to see if state vaccination rollouts sputter out, or if children are vaccinated at high rates as shots for younger age groups come available.
“The more people we get vaccinated, the higher that benefit will be,” she says. “We want to get back to a place where people can go to a big concert and not worry, where schools can be open when they’re distancing or masking. That’s the goal—to be able to get back to no worrying.”