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Ciara O'Rourke
By Ciara O'Rourke August 7, 2020

No, the U.S. hasn’t achieved herd immunity from COVID-19

If Your Time is short

We have not reached herd immunity against the coronavirus, and scientists say we aren’t close to achieving it in the United States.

Scientists estimate herd immunity will probably be reached when 60% to 70% of the population has been infected. 

Sharing a graph of COVID-19 testing trends in the United States, a recent Facebook post claims that "HERD IMMUNITY HAS BEEN REACHED."

"In fact," the post says, "it was probably reached in May."

The post points to two "bumps" in the data. The first bump, it says, reflects when at-risk patients were tested for COVID-19 in hospitals. The second bump reflects when testing became more available to more people, according to the post. 

"We’re looking at the downward trend of that second bump," the post says. "Ask any biostatistician what it means when you test hundreds of thousands of people a day for an infectious disease and return a consistently lowering positivity rate. I’ll give you a clue: it means heard immunity has been reached and the virus is dying out." (The misspelling of "herd" is in the original post.)

This post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.) 

We’ll cut to the chase before drilling down into the details: No, we have not reached herd immunity, a subject we’ve written about before

Scientists generally say herd immunity could play a role in curbing the spread of the coronavirus before a vaccine becomes available. The idea is that once a disease infects enough people in a community, if the infection confers immunity, then there are fewer people left who can still transmit the disease, so the spread slows. For the coronavirus, scientists estimate herd immunity will probably be reached when 60% to 70% of the population has been infected. 

The graph featured in the Facebook post shows daily changes in COVID-19 tests performed in the United States and test positivity rates over the months. It’s available on Johns Hopkins University’s website

But Jennifer Nuzzo, an epidemiologist at Johns Hopkins, said it "cannot in any way be used to assess whether or not we’ve achieved herd immunity." The Facebook post, she said, is a "completely inappropriate interpretation of what it’s showing." 

Rather, she said, test positivity is a measure specialists use to gauge whether a community is conducting enough testing. Because the rate of people testing positive for COVID-19 can change depending on the number of tests administered, it doesn’t necessarily reflect how many people are becoming infected. Low positivity rates, for example, could result from simply testing a large number of uninfected people, she said. Plus, asymptomatic people who are infected and possibly spreading the disease may never get tested.

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Scientists use serological surveys of a representative sample of the population to estimate what fraction of a community has likely been infected with COVID-19, Nuzzo said. The Centers for Disease Control and Prevention has done this, and according to its most recent findings, "no one is near herd immunity, even in the hardest-hit places in the United States."  

"Other countries, including places like Sweden that have not implemented the same level of shutdowns like the United States — their surveys say the same thing: that the vast majority of us remain susceptible to this virus," she said. 

We contacted several other specialists for help parsing the claims in the Facebook post. We even asked a biostatistician, as the post suggested.

"This post is not accurate," said Natalie Dean, a biostatistics professor at the University of Florida. 

Because test positivity is low in some areas, like New York, and high in others, like Florida, it’s "not particularly meaningful to look at the entire United States as one unit," she said. "While numbers in some of the hotspot states appear to be stabilizing, it is hard to know for sure, because testing capacity is strained and there are significant backlogs."

That makes daily positivity numbers less reliable. Still, she said, decreasing 911 calls for respiratory distress in some cities and indicate that the spread of COVID-19 is slowing down. That’s likely due to policy and behavioral changes in different communities.

But broadly in the United States, COVID-19 is "spreading like wildfire," said Andrew Noymer, a population health and disease prevention professor at the University of California-Irvine. 

When testing increases, the percentage of positive tests can decrease simply because more people are being tested, he said. "So percent positive testing going down doesn’t mean anything."

The Facebook post, he said, "is just a bunch of mumbo jumbo."  

Jaquelin Dudley, a professor of molecular biosciences at the University of Texas at Austin, said it’s possible there are areas where herd immunity has been established, "but there are still many unknowns, including lack of testing for asymptomatic individuals or missing the right point during infection for testing."

At this point, though, we can’t accurately predict a timeline for when there will be herd immunity throughout the United States, she said. 

We rate this Facebook post False.

Our Sources

Facebook post, Aug. 4, 2020

Johns Hopkins University, Daily state-by-state testing trends, visited Aug. 6, 2020

Interview with Andrew Noymer, associate professor, population health and disease prevention, University of California Irvine, Aug. 6, 2020

Email interview with Jaquelin Dudley, professor of molecular biosciences, University of Texas at Austin, Aug. 6, 2020

Email interview with Natalie Dean, professor of biostatistics, University of Florida, Aug. 6, 2020

Interview with Jennifer Nuzzo, epidemiologist, Johns Hopkins University, Aug. 7, 2020

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No, the U.S. hasn’t achieved herd immunity from COVID-19

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