During a live Instagram interview, Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said that depending on herd immunity to end the coronavirus pandemic would let to an "enormous," "totally unacceptable" number of deaths.
"If everyone contracted (COVID-19), even with the relatively high percentage of people without symptoms ... a lot of people are going to die," Fauci, the nation's top infectious disease expert, told actor Matthew McConaughey on Thursday, citing pre-existing conditions that might make people more susceptible to severe coronavirus reactions.
"You look at the United States of America with our epidemic of obesity, as it were," Fauci said. "With the number of people with hypertension. With the number of people with diabetes. If everyone got infected, the death toll would be enormous and totally unacceptable."
Herd immunity is defined as a population reaching a point where enough people have become immune to a disease that it can't spread effectively. That immunity can be achieved either with a vaccine, or by letting enough people become ill with and recover from the illness. It's estimated that for COVID-19, between 60 and 70% of the population would need to get the virus for herd immunity to be achieved.
In the past, experts have been skeptical about herd immunity being an effective solution to the coronavirus crisis. Antibodies may not confer immunity, and there's no definitive data showing how long a person is immune from the virus after recovering from it. To complicate matters further, antibody tests can be flawed, giving misleading information about the rates of antibodies present in a population.
"If natural immunity to this virus is going to be gone within three to six months, then we don't even need to be talking about herd immunity," Dr. David Dowdy, an associate professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health, told NBC News in May.
Previous research shows low rates of antibodies even in hard-hit populations. Preliminary testing of 1,300 New York City residents found that just 21.2% were positive for antibodies.
"That means 80 percent of the population would appear to still be susceptible," Dr. Robert Atmar, an infectious disease specialist at Baylor College of Medicine in Houston, told NBC News in May. "So even in an area that has been highly affected, we're not seeing the levels that you would expect for herd immunity."
Even countries that didn't lock down had mixed results with achieving herd immunity. Sweden instituted no lockdown or social distancing mandates, leading to thousands of deaths and tens of thousands of confirmed cases, but in late April the country's public health agency said that just one-third of the residents of the country's capital city had tested positive, a far cry from the 70% immunity estimated to be necessary.
Dr. Marybeth Sexton, an infectious disease expert, hospital epidemiologist and assistant professor of medicine at Emory University in Atlanta, told TODAY in June that there would be the potential for millions of deaths if herd immunity became a strategy in the U.S.
"The problem with “natural” herd immunity — meaning you let the disease spread — is that if you assume COVID-19 has a mortality rate of about 1% and you think about 60-70% of the U.S. population getting it, that’s more than 200 million people, so that’s more than 2 million deaths to get to herd immunity," Sexton said. It's also expected that the effects of herd immunity would lead to overstretched hospitals and people not being able to get care for other conditions, she said.
Instead, the safer solution is to wait for a vaccine, which can help confer immunity without the massive death toll experts fear. Until then, it's advised that people continue to follow precautions like social distancing, mask-wearing and frequent hand washing.
“Absolutely we should have universal wearing of masks," Fauci told McConaughey.