As physicians and scientists around the world race to find a cure or drug-based treatment for COVID-19, a persistent few voices have championed herd immunity as a way to both shut the door on the virus and open the economy.

In the United Kingdom in the early days of the pandemic, herd immunity was at first advocated by Prime Minister Boris Johnson before his government abandoned it, and he himself later contracted the virus. Although proceeding with extreme caution, Sweden has imposed only limited restrictions on movements, and its government believes the capital of Stockholm may achieve herd immunity this month.

But what exactly is herd immunity and why do many, if not most, scientists, as well as ethicists, raise questions and concerns about the concept?

UT Health San Antonio’s Carlos Roberto Jaén, M.D., Ph.D., professor and chair of family and community medicine as well as an epidemiologist, said it’s all about protection, but with the ultimate danger.

Herd immunity is “the protection that a group or community can provide to you in the context of an infectious disease,” he said. “In other words, when a proportion of a population is immune to an infectious disease, there is an indirect protection to those who are not immune. For example, if you have 100 people and 80 of them are already immune, then you have only a one in five chance of becoming infected if you come in contact with them because 80% of your contacts will not have it.”

But if 70% of a population is immune as opposed to 80%, your risk goes up, he said.

There are two ways to get a population to a high level of immunity, Dr. Jaén said. One way is through developing a vaccine and immunizing most of a population. The other, more problematic, is for enough people to get the infection, perhaps by letting it spread, and then recover.

“In the context of COVID-19, which has no vaccine, intentionally exposing people is not a good idea,” said Dr. Jaén, also a member of the National Academy of Medicine. “One, the infection is very efficient in how it passes from one person to another. Two, there would be a significant number of people in our community who would have a very severe infection. It would overwhelm the health care system, and there would be many deaths.”

Moreover, said Dr. Jaén, there is too little known about how the coronavirus would move toward achieving herd immunity in the absence of a vaccine. How the disease is spread, the reproduction number, how it attacks the body, the percentage of the population needed to reach herd immunity, even whether a person can be re-infected after recovering—all of these are questions.

It is known, he added, that “the death rate for COVID-19 is about 10 times higher than the flu.”

“Herd immunity is a phenomenon, it’s not really a method,” Dr. Jaén said. “The rampant exposure—like let’s just have a COVID party the way they used to have chicken pox parties to get it over with—the problem here is the disease is so severe, whereas with chicken pox you get a few poxes and you’re a little sick for a few days. For COVID-19 for you may be intubated, not able to breathe and may die.”

That’s why social distancing, masking, staying in place, working remotely—“all those measures are designed to minimize the rate at which people are getting infected,” he said.

Amelie Ramirez, Dr.P.H., chair of the Department of Population Health Sciences and founder and director of the Institute for Health Promotion Research at UT Health San Antonio, echoed Dr. Jaén’s concerns and urged a new understanding of herd immunity.

“Part of this is it’s exposing the inequities in our health care system,” she said. “We’re seeing a disproportionate number of our underrepresented communities being impacted by COVID-19, such as African Americans, Hispanics and Native Americans. Access is a big issue. We have unequal access to health care for a large portion of these individuals. They are less likely to have health insurance, less likely to have sick leave. So economic status does play a role.”

Dr. Ramirez, also a member of the National Academy of Medicine, pointed to a May article in the Journal of the American Medical Association calling on the health care industry “to redefine herd immunity.”

Quoting from JAMA, Dr. Ramirez said, “The U.S. must develop a new kind of ‘herd immunity,’ whereby resistance to the spread of poor health in the population occurs when a sufficiently high proportion of individuals, across all racial, ethnic and social class groups, are protected from and thus ‘immune’ to negative social determinants.”