Don’t expect herd immunity to rescue the U.S. from the COVID-19 pandemic anytime soon. The country needs about 200 million infected and immunized citizens before the chain of infection can be broken. And the numbers just don’t add up, according to James L. Madara, MD, executive vice president and CEO of the AMA.

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Herd immunity exists when so many people have been infected and become immune—or immunized by vaccination—that the chain of infection is broken. But to achieve this dynamic, a realistic estimate is that about 70% or more of the population needs to gain immunity to SARS-CoV-2, Dr. Madara explained during a recent episode of the “AMA COVID-19 Update.”


“We're nowhere near that,” he explained. The Centers for Disease Control and Prevention (CDC) has estimated that only about 9% of the U.S. population has actually been infected and is immune. That’s about 30 million people, and even if that count doubles in six months, that’s only about 20% of the population,

“We'd need about 230 million Americans with an immunity, and that's 170 million more than that 60 million that we estimated from the beginning of the pandemic … six months from now,” said Dr. Madara. Vaccines are a way to achieve that number, but vaccines aren’t perfect in their ability to deliver immunity.

“Vaccines, and specifically those for airway viruses, aren't 100% effective, and that's why both the FDA [Food and Drug Administration] and the CDC—while they would hope for a vaccine that might be, say, 70% effective—they have set the acceptable target at 50%.” Dr. Madara explained.

So, to achieve the 130 million more immune individuals, nearly all of the remaining population would have to be immunized, to get the right amount for herd immunity. And there is always a significant portion of the population that refuses vaccination.

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“This makes the math even more daunting. So, for example, if 20% of the population refused the vaccine, we're now out of the range that gets us to herd immunity,” said Dr. Madara.

Prior to arriving at the AMA, Dr. Madara spent the first 22 years of his career at Harvard Medical School, serving as a tenured professor and as director of the National Institutes of Health-sponsored Harvard Digestive Diseases Center. Dr. Madara also served as dean of the medical school and CEO of the hospitals at the University of Chicago. Having published more than 200 original papers and chapters, Dr. Madara has received both national and international awards, and served as editor-in-chief of the American Journal of Pathology and as president of the American Board of Pathology.

In his interview with AMA Chief Experience Officer Todd Unger, Dr. Madara explained that researchers are also not certain about how long an immune response lasts after infection or vaccination. For other infections from the coronavirus family, immunity lasts from a few months to a couple of years, Dr. Madara says. Because COVID-19 is a novel coronavirus, it is difficult to be certain. “We would guess it would not be lifetime immunity,” he said, so at some point, individuals can get reinfected.

Failure to achieve true herd immunity means that despite everyone’s hope that the pandemic can be defeated as soon as possible, struggles will continue.

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“We wondered back in March and April if we would be back by the summer,” Dr. Madara said. “Didn't work out that way. We will continue to have struggles, and there are no magical solutions—and that includes vaccines. We can't imagine when there's a vaccine that we're ... back to normal now. That's unlikely to be the case.”

In the meantime, Dr. Madara said researchers need to continue to pursue treatments, such as monoclonal antibodies, and people need to continue to try to control the spread with masking, hand washing and social distancing. Learn more with the AMA about why it’s critical to mask up to stop the spread of COVID-19.

“It's also important that we are transparent in our vaccine development. So, when a vaccine is approved, physicians feel confident in recommending it, and patients can feel confident in getting it,” he said.

Discover why trust in the coronavirus vaccine starts with transparency.

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