Could Americans Get to COVID Herd Immunity by Late Spring?

WEDNESDAY, Feb. 24, 2021 (HealthDay News) -- Hungry for good news on the pandemic? One epidemiologist believes Americans might reach herd immunity to the new coronavirus as soon as late spring.

That's the view held by Suzanne Judd, a professor with the school of public health at the University of Alabama (UA) at Birmingham. To come to that conclusion, she reviewed recent research and data from her home state.

"I really am starting to feel like we are seeing the edge of the woods, like we are seeing the light at the end of the tunnel," Judd said during a Wednesday media briefing.

According to the U.S. Centers for Disease Control and Prevention, herd immunity occurs when enough people in a community are protected from getting a disease because they've already had the disease or because they've been vaccinated.

Scientists largely believe "enough people" means 72% of the American public developing immunity to the coronavirus, Judd said, though she cautions that "there is no magical number that is herd immunity."

Still, she noted that trends unfolding in the state of Alabama suggest that herd immunity is now on track to take hold by late spring or early summer.

Judd's optimistic reading is based on two main considerations. One is the rate by which Alabama residents are getting vaccinated with a second dose, which she puts at roughly 50,000 people per week, a figure she calls a "conservative estimate."

Another is the large number of people who likely already have coronavirus antibodies, despite not having tested positive.

For example, Judd said that a recent Columbia University study led by infectious disease specialist Jeffrey Shaman (which still awaits peer review) reveals that the actual number of Americans who've been infected is likely five times higher than what's been reported since the start of the pandemic.

The Columbia team also puts the true number of active COVID-19 cases as perhaps 10 times as high as official numbers indicate, which Judd said is probably due to insufficient testing.

Even so, Judd's optimistic take on Alabama's prospects for springtime herd immunity presumes a far lower undercount. Based on 500,000 confirmed diagnoses in Alabama to date, Judd pegs the true statewide caseload at 1.5 million, reflecting all those who have "never even tested positive for the virus, and some of whom may never even have had symptoms."

Another concern is the potential for future viral mutations. But not all mutations undermine acquired immunity, Judd stressed. And in the event some do, the response will be vaccine adjustments and boosters, "something we do all the time with the flu."

With all that in mind, herd immunity will take hold when the number of new weekly infections falls to (and remains at) five or 10 cases per 100,000 individuals or less. "Then we'll know it's safe to get people back together," Judd said.

"True herd immunity would allow us to start getting back together again," she said. "The problem was that [the new coronavirus] sent a lot of people to the hospital and killed a lot of people."

But a virus that becomes "less virulent, less strong," will be another sign of herd immunity.

Dr. Amesh Adalja is a senior scholar with the Johns Hopkins Center for Health Security, in Baltimore. Though not part of the UA briefing, he thinks that, at this point, "approximately one-third of the population likely has some immunity from natural infection, and a smaller proportion from vaccination." That's the national figure floated by the Columbia University team.

Adalja agreed that "it is true that the level of population immunity that the United States currently has is impacting the spread of coronavirus." Yet, "it will still be some time before herd immunity thresholds are crossed," he cautioned.

"However, I do not think that is the only thing that matters," Adalja added. "As vulnerable populations are vaccinated, we will see pressure on hospitals lift, and that will change the risk perception of this disease."

Meanwhile, Judd warned that until herd immunity takes hold, it's premature to stop following CDC guidance on social distancing.

"It would definitely be an issue if people stop wearing masks before the virus has been brought under control," she said, noting that it remains to be seen whether or not a vaccine blocks transmission. "That's why even if you've been vaccinated, you should still wear a mask," Judd advised.

More information

There's more on COVID-19 and herd immunity at the CDC.

SOURCES: Suzanne Judd, PhD, epidemiologist and professor, school of public health, The University of Alabama at Birmingham; Amesh Adalja, MD, senior scholar, Johns Hopkins Center for Health Security, Baltimore; University of Alabama, media briefing, Feb. 24, 2021