The term “herd immunity” has been floating around in headlines for weeks. Sweden’s ambassador to the United States says Stockholm will reach it for COVID-19 by the end of May. Some people are considering intentionally exposing themselves to the virus—against the advice of health experts—to achieve it. But what is herd immunity? Brian Labus, PhD, MPH, an infectious disease epidemiologist and public health professor at the University of Nevada, Las Vegas, explains that for COVID-19 it would mean the broader population is protected from widespread infection—but that’s unlikely to happen anytime soon in the U.S.
“Herd immunity is the idea that there is a high enough level of immunity in a population that disease transmission cannot be sustained and an outbreak would quickly extinguish itself,” says Dr. Labus. “If we reach it in the population, it would mean we have the coronavirus outbreak under control.”
Dr. Labus explains that there are two ways to reach herd immunity: natural mass infection and vaccination. “The level of immunity needed in the population depends on how easily the disease spreads,” says Dr. Labus. For example, the population of the United States has herd immunity to the measles (largely due to vaccination), which, because it’s so highly contagious, means that at least 93 percent of the population is immune. For COVID-19, 60 to 65 percent of the population will likely need to be immune, says Dr. Labus. But Gregory Poland, MD, who studies the immunogenetics of vaccine response at the Mayo Clinic, warns that it’s too early for herd immunity to impact the spread of COVID-19. “There’s no chance that immunity is going to be high enough to reach herd immunity,” he tells MarketWatch, adding that he does not believe a will be available vaccine by next winter.
In an interview with Time, director of the National Institute of Allergy and Infectious Diseases Anthony Fauci, MD, explains that the U.S. is nowhere near the point of herd immunity. “We really can’t depend on herd immunity until we get either enough people infected, or enough people vaccinated,” he says.
“Immunity to a disease is achieved through the presence of antibodies to that disease in a person’s system,” explains the Centers for Disease Control and Prevention. “Antibodies are proteins produced by the body to neutralize or destroy toxins or disease-carrying organisms.” Once a person immune to a disease (either by infection with the disease itself or through vaccination), the human body recognizes it when exposed to it and produces the antibodies needed to fight it.
Antibody tests have been touted as a means to determine who has already been infected and therefore potentially immune to COVID-19, but doctors are learning that many tests are inaccurate. Last week, Chile announced plans to issue “immunity passports”—essentially permission slips allowing those infected and recovered to go back to work—but scientists question the durability of immunity as the World Health Organization cites no evidence that recovered COVID-19 patients are immune to the disease. (The United Kingdom, Germany, and Italy are also exploring the idea.)
“We don’t know if immunity to COVID-19 is temporary or long-lasting,” says Dr. Labus. “If people lose immunity within a few months of infection, it would be difficult to ever reach [herd immunity].”
Before vaccines became widely available, it was difficult if not impossible to achieve high levels of immunity for infectious diseases. “Diseases would never really disappear and would cause outbreaks every few years,” says Dr. Labus. “Unfortunately, the only way to reach the needed levels of immunity right now is through natural infection. If over 60 percent of our population becomes infected, we will have many people die from this disease before the outbreak is over.”
And even if we are able to reach herd immunity for COVD-19, that doesn’t mean that the disease will cease to exist.
“It is a common misconception that herd immunity will prevent disease altogether,” says Dr. Labus. “Even if there is herd immunity, individual people will become sick, and there could be an outbreak among a subset of the population—people who aren’t immunized, for example.”
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