Herd Immunity Is Not a Magical Percentage
For months, a tantalizing number has driven America’s Covid-19 response. First, it was 60 to 70 percent; then, as the virus blanketed the globe and evolved to become more contagious, it rose to 75 or even 85 percent.Herd immunity, which occurs when enough people become immune to a virus that new cases don’t take off and cause outbreaks, has been a major goal in the United States since the pandemic began. Exactly what percentage is needed in the case of SARS-CoV-2, the virus that causes Covid-19, is unclear, but the latest estimate is at least 70 percent. And the key to reaching it, experts say, is paradoxical: Don’t focus on it too hard.While the Trump administration last year misused the concept of herd immunity to propose letting the virus rip through the population—something epidemiologists called “barbaric”—the development of effective vaccines seems to have finally put the goal within reach. In theory, if enough people get vaccinated, we can finally tame Covid-19.Lately, vaccinations across the country have faltered. The hockey-stick graph of record-setting daily shots reached a peak in mid-April and then began falling. Last month, a New York Times story shocked readers by reporting that some experts now think we won’t reach herd immunity because not enough people will be vaccinated. But herd immunity—or, as experts prefer to call it, community immunity—can still be reached; it will just take a little time and a lot of effort.The key is not to make herd immunity our only goal. We need to make sure all communities across the country are protected equitably, and we need to continue efforts to vaccinate the world. Otherwise, we could reach widespread immunity and then lose it, if the virus evolves until current vaccines are less effective. “Even if you put out a raging fire in your own backyard, and there’s fire around all around you, you have a really high chance of embers finding their way and starting the fire again,” Dr. Saad Omer, director of the Yale Institute for Global Health, told me.About 76 percent of U.S. adults have either gotten their first shot or plan to, according to a Gallup poll released this week. Another 5 percent say they don’t plan on it, but they’re open to changing their minds. According to the nonpartisan Kaiser Family Foundation, which has tracked attitudes toward the vaccines throughout the pandemic, only 13 percent of Americans would outright refuse the vaccine, while another 7 percent would only get it if required to—which will likely become the norm at schools, workplaces, travel hubs, and more. The Pfizer-BioNTech vaccine was recently authorized for emergency use among children above the age of 12, and clinical trials in younger children are now underway. Plus, the natural immunity acquired by Covid-19 survivors also helps stem the swell of new cases. With all of that in mind, community immunity is well within sight in the U.S.Although the slump in vaccinations seems alarming, for many epidemiologists, it was entirely predictable. A few weeks ago, news organizations were updating graphs with vaccination rates rising upward, moving steadily toward the goal of herd immunity. But when I talked to Dr. Justin Feldman, a social epidemiologist and a fellow at the Harvard FXB Center for Health and Human Rights, back in April, he was already predicting access issues would soon slow the meteoric growth. “I think it’s going to curve downward,” he said. “Because at some point, the people who are going to get themselves to these vaccination sites are going to be exhausted, and you’re left with a pretty large group of people who are willing but need extra work to get there.”That doesn’t mean all is lost, he and other experts told me. It just means what health experts have always known: Ending the pandemic will involve reaching those who need help the most but don’t always have the resources, time, or knowledge to get it. This is the biggest vaccination campaign in human history, so it’s not going to happen on autopilot in a matter of weeks. But widespread protection can happen, as long as we continue to focus on reaching everyone. And we need to continue taking other preventative measures as well, in order to protect 100 percent of people.Herd immunity is “not a magic number,” where you reach a finish line and the disease is gone, Omer said. There will still be cases, he said, but because there are enough immune people to keep the sparks from catching fire, “they fizzle out.”“People misunderstand the concept as a target which, once attained, will permit full return to normal. That’s not the way it works.”Dr. William Hanage, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health, told me in an email that “people misunderstand the concept as a target which, once attained, will permit full return to normal. That’s not the way it works.” Instead, community immunity is an ongoing goal, something we won’t realize we have reached until we have passed it—and, crucially, something we need to maintain through the years with booster shots and the vaccination of young children, once the vaccine is approved for their use. It’s an ongoing process, not one and done. As Omer put it: “It’s not a cruise-control situation. We need to work toward it.”Instead of aiming single-mindedly for herd immunity, we need to focus on preventing all new infections, in order to keep the vaccines effective and keep people from dying. “You can do that with vaccination, immunity from prior infection, and things like masks. As we get more of the first of those, the second two need to do less of the work, and we end up with fewer sick or dead, and less restrictions in our daily lives,” Hanage wrote.When it comes to vaccination, one strategy is focusing on those who are at highest risk of getting and spreading the virus. There’s now good evidence that the vaccines being used in the U.S. help reduce transmission as well as protecting the vaccinated person. “By vaccinating people who have the most social contacts and at highest risk of transmission, you are making it more difficult for the virus to spread,” Feldman said. Many people within this group are frontline workers who tend to have trouble taking time off work to get the shot, which means vaccinators should come to workplaces and communities instead of waiting for people to come to them.Smart, targeted approaches like these could help make a significant dent even before we reach community protection, Omer said. “You still see things catching fire, but not at this level, the same frequency and intensity. The bottom line is, if you’ve been trying to achieve herd immunity, it’s still a worthwhile goal. But you will get a lot of benefits short of that.”National figures can also obscure striking disparities across states and within communities. States like Vermont, Maine, and Massachusetts have vaccinated more than half of their entire populations, while fewer than 30 percent of all residents in Mississippi and Alabama have gotten shots. That means some parts of the U.S. will reach community immunity long before others, while the pandemic still rages among unvaccinated people. As tempting as it is to believe the pandemic is ending, it won’t as long as there are communities not being reached by public health efforts.“If we’re going to talk about why are we having such a difficult time getting to a higher percentage of people vaccinated in the United States, we should at least talk about what happened last year.”Omer, who has long researched the reasons why people don’t get vaccinated, doesn’t like to use the phrase “vaccine hesitancy” to describe those who haven’t been vaccinated yet. Instead, he talks about the social and behavioral reasons underpinning how people make health decisions. Some people who can’t or don’t want to go to get vaccinated should have the vaccines brought to them; those who have questions about the vaccine’s safety or efficiency should have those questions addressed in straightforward, thoughtful ways. The vaccines are “incredible,” Dr. Cecília Tomori, director of global public health and community health at the Johns Hopkins School of Nursing, told me. But innovations like these alone can’t solve complex social problems. “If we’re going to talk about why are we having such a difficult time getting to a higher percentage of people vaccinated in the United States, we should at least talk about what happened last year,” she said. The pandemic was highly politicized, and hundreds of thousands of Americans died in the pursuit of natural immunity, which had “very clear eugenic undertones,” Tomori said. Trust needs to be rebuilt, and “prevention,” through masks, distancing, and testing, “is still really, really important,” she said.Reaching community immunity is great, she said, but some political leaders and journalists have perhaps emphasized it too much. “Maybe we’ve put too much focus on that, and maybe there are other ways to reach it and other things to be thinking about,” she said—such as improving access for those who want the vaccine but haven’t gotten it yet, as well as protecting frontline workers with paid sick leave, workplace protections, and housing support so they can isolate if they do get sick.Focusing only on vaccinating communities in the U.S. is dangerously shortsighted, experts say. “It is increasingly clear that if our focus is solely or even mainly domestic, inward-looking, we leave ourselves vulnerable to newer variants; we leave ourselves vulnerable to importation of this virus,” Omer said. The U.S. needs to focus “not as a side activity, not as a few people … with bleeding hearts—but as a national priority” on vaccinating the world, he said. “We are sleepwalking into a situation where it increases our vulnerability. And it increases the chance that we will be judged harshly by history.”Widespread immunity is within reach. But in focusing on that magic vaccination percentage only in the U.S. and ignoring the other tools needed to slow transmission, we risk prolonging the epidemic and pushing the goal further away than ever. Community immunity will be reached when we focus beyond it—on protecting everyone who is at risk of getting sick and passing the virus on.
Read More
