- A population can achieve herd immunity after enough people become immune to a virus, either through vaccination or infection.
- Arizona, Florida, and Texas — which had huge coronavirus outbreaks following early reopenings — may have reached low levels of herd immunity that have helped slow the virus's spread in the last month.
- But that came at the cost of tens of thousands of lives, and experts say it's still not enough to prevent further outbreaks if these states go back to normal life as it was pre-pandemic.
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So many Americans have gotten COVID-19 that their immunity may have helped slow the virus' spread in key hotspots.
A lion's share of the infections and deaths during the US's second coronavirus surge this summer were clustered in a handful of states that reopened quickly, including Arizona, Florida, and Texas. Since then, however, new cases in those states have subsided significantly, dropping almost to the levels reported before the summer wave.
According to epidemiologist Trevor Bedford, this could be partially because so many people in those states have developed immune responses, so the virus now has more difficulty spreading. This, in essence, is a degree of herd immunity: the point at which enough people become immune to a virus to significantly limit its ability to spread.
The safe way to gain herd immunity is through mass vaccination. The dangerous way is through mass infection — but that's usually what people mean when they talk about a "herd immunity strategy" in this pandemic.
"I certainly believe that the 'herd immunity' strategy for dealing with COVID-19 is hugely overly costly in terms of health impacts," Bedford wrote on Twitter last month. "But it does seem like the strategy is being perhaps unintentionally pursued in parts of the US."
Now, a month later, it seems states like Arizona, Florida, and Texas have stumbled upon the beginnings of herd immunity — which may be playing a role in the states' low daily infection tallies. But it came at a huge cost: Since May 15, at which point all three states had begun reopening, the coronavirus has killed more than 4,600 people in Arizona, more than 10,600 in Florida, and more than 12,700 in Texas.
It's unclear what infection rate leads to herd immunity
Epidemiologists don't know what coronavirus infection rate it takes for a population to reach herd immunity. Generally, this proportion gets determined based on the number of people to whom an average infected person passes a pathogen. For the coronavirus, that's about two to three, meaning that about 50-70% of the population would need to be protected via vaccination in order to stop the virus' spread.
But when it comes to immunity gained as a result of infection, the dynamics of how people move and behave affect the herd immunity threshold. Some groups of people have more interpersonal contact than others, for example. So if large portions of those particular groups contract the virus and gain immunity, that could significantly decrease future opportunities for the virus to spread.
"Young, healthy people are more likely to get immunity and 'cover' the rest of the population," Youyang Gu, a computer scientist who created the model covid19-projections.com, told Business Insider.
For those reasons, some research has suggested that if 10-20% of a population gets infected, that could bring about some level of herd immunity, since the virus would mostly have spread among people who have the most contact with others.
Many researchers think the herd immunity threshold is therefore somewhere between 20% and 70%, and that it will vary for different populations.
"I think data we have now (that is robust) indicates it is >25%," Marm Kilpatrick, a disease ecologist and biostatistician at the University of California Santa Cruz, told Business Insider. "But it could be as low as 30%. It might be as high as 50% in some [populations]."
Levels of immunity below these thresholds can still help curb the virus's spread, though, even if they don't eliminate it.
Partial herd immunity may have helped flatten the curve in some places
New York City, where the first major US COVID-19 outbreak overwhelmed hospitals and morgues in March and April, seems to also be the first place in the country to have gained a significant degree of immunity.
A survey from the Centers for Disease Control and Prevention detected coronavirus antibodies — indicating an immune response — in 23% of New York City blood tests in late April and early May.
Similarly, Gu's model estimates that about 25% of Florida's population had been infected as of Friday. In Arizona, his estimates suggest the portion is 24%, while it's around 23% in Texas, and almost 21% in New York state.
However, that's not the only reason for the decline in new cases in those states: Epidemiologists also attribute the trend to behavioral changes like mask-wearing, social distancing, and new business closures.
"Disentangling behavioral effects from herd immunity is very difficult," Kilpatrick said.
That's because in addition to limiting the virus's opportunities to spread, these behavioral changes also reduce the level of herd immunity needed to curb an epidemic.
"Because of this reduction in transmission through social means, we don't need as much immunity to impact spread," Bedford said.
But he doesn't think these immunity levels would be enough to stop the virus from spreading if people go back to normal life.
Although the outbreaks may "leave enough immunity to assist in keeping COVID-19 controlled" for now, Bedford said, "this level of immunity is not compatible with a full return to societal behavior as existed before the pandemic."
Herd immunity is not a good game plan
A couple weeks ago, The Washington Post reported that Scott Atlas, a neuroradiologist serving as a top pandemic adviser to the Trump administration, had urged the president to pursue herd immunity by reopening businesses across the country and allowing the virus to spread among young and healthy people, while keeping the elderly and vulnerable at home.
Atlas denies this, however: "There is no policy of the president or this administration of achieving herd immunity," he said in a statement. "There never has been any such policy recommended to the president or to anyone else from me."
The cost of pursuing a herd-immunity strategy would be unconscionable, according to vaccinologist Florian Krammer.
"The only way to achieve herd immunity is through vaccination. Everything else is ethically questionable," he told Business Insider. "Would you take a vaccine that kills 1% of people who get vaccinated and protects the remaining ones — but of the remaining ones, 90% get sick? That's what infection does."
Even in an optimistic scenario in which only 20% of the population needs to get infected to reach herd immunity, about 600,000 people would still die in the US.
That's not to mention all the people who would fall ill — many so severely that they would end up in the hospital. Plenty of them would develop debilitating long-term symptoms: fatigue, difficulty breathing, and kidney damage to name a few.
"Humans are not herds," Michael Ryan, Executive Director of the World Health Organization's Health Emergencies Program, said in May. "I think we need to be really careful when we use terms in this way around natural infections in humans, because it can lead to a very brutal arithmetic which does not put people and life and suffering at the center of that equation."
Indeed, the US has paid a steep price for just a few states to get a small degree of herd immunity through natural infection: mass death, overwhelmed hospitals, refrigerator trucks filled with bodies, and nationwide testing shortages.
"The costs for this immunity have been substantial and are continuing to accrue," Bedford said. "We need a vaccine to achieve population immunity in a fashion that doesn't kill people."
Sinéad Baker contributed reporting to this story.
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