Here's Why Herd Immunity Is Now "Not a Possibility," Virus Expert Says

One expert said the idea of achieving herd immunity is "mythical."

Before the public even started to get vaccinated against COVID, experts had been emphasizing the importance of achieving herd immunity in order to finally end the pandemic. The concept refers to the phase in which enough people in a population are protected from a virus, either due to natural infection or vaccination, so that it can no longer spread. However, now, it's looking more and more like herd immunity may not be possible, due to the highly contagious Delta variant that's become the most dominant strain of COVID in the U.S. But it's not necessarily the variant's increased transmissibility that's the problem, according to experts.

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Earlier on in the pandemic, White House COVID adviser Anthony Fauci, MD, said he believed herd immunity would be achieved when about 70 percent of the population had been infected by COVID or got vaccinated, but he continuously raised the percentage as time went on. In April, The New York Times reported that Fauci landed on the idea that somewhere near 90 percent of people would need to get COVID or their shots in order to achieve herd immunity. In May, as it became clear vaccination rates were slowing, the conversation started to change. "People were getting confused and thinking you're never going to get the infections down until you reach this mystical level of herd immunity, whatever that number is," Fauci told The New York Times. "That's why we stopped using herd immunity in the classic sense."

Enter the Delta variant, which has changed the course of the pandemic for the worse. "I think we are in a situation here with this current variant where herd immunity is not a possibility because it still infects vaccinated individuals," Sir Andrew Pollard, PhD, head of the Oxford Vaccine Group, said during a U.K. parliamentary meeting on Aug. 10. He added that since the vaccines don't stop the spread of the virus entirely, the idea of achieving herd immunity is "mythical."

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Pollard explained that COVID is different than other viruses we vaccinate against, which is part of what makes achieving herd immunity so challenging. "The problem with this virus is [it is] not measles," he said. "If 95 percent of people were vaccinated against measles, the virus cannot transmit in the population."

While the current vaccines can help slow the spread of the virus, new variants are proving they can still lead to an increase in transmission, Pollard said. "I suspect that what the virus will throw up next is a variant which is perhaps even better at transmitting among vaccinated populations, and so that's even more of a reason not to be making a vaccine program around herd immunity," he said.

On Aug. 12, Andrew Freedman, MD, infectious disease expert at Cardiff Medical School, told CNBC that he also thinks that herd immunity was unlikely. "The Delta variant is highly transmissible, meaning that the proportion of people needing to be fully vaccinated for herd immunity is probably not achievable," said Freeman.

This is especially true as only 59 percent of the eligible population is fully vaccinated in the U.S., according to data from the Centers for Disease Control and Prevention (CDC). "The vaccines provide very effective protection against severe disease/hospitalization/death but are less effective in preventing infection, mild disease and transmission, especially for the Delta variant," added Freedman.

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