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Fact check: Rate of positive COVID-19 tests doesn’t show U.S. has reached herd immunity

A Facebook post shared over 3,600 times misleadingly alleges the current rate of positive COVID-19 tests in the U.S. indicates that “herd immunity has been reached”. This is not the case.

Reuters Fact Check. REUTERS/Axel Schmidt

The lengthy post is visible here . It claims: “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 (sic) immunity has been reached and the virus is dying out.”

This fact check will focus on that primary claim, not on the remaining content of the post.

WHAT IS HERD IMMUNITY?

Herd immunity, also referred to as herd protection, describes a scenario where enough people in a population have immunity to an infection to be able to effectively stop that disease from spreading. This can be achieved either through vaccination, which is still not available for COVID-19, or from people having had the disease. (See a Reuters explainer here ).

Experts estimate that to achieve herd protection from the new coronavirus through infection, at least 70% of the population in the U.S. would need to have recovered from the disease and developed antibodies against future infection ( here , here ). Recent projections range between 43 and 66% ( bit.ly/3koj8WY , bit.ly/2Do75Z8 )

But there are still questions about whether the antibody response to the virus is robust and how long it lasts ( here ). Emerging evidence suggests that the body’s antibody response against COVID-19 may only last months after infection ( here ).

WHAT DOES THE RATE OF POSITIVE TESTS MEAN?

The claim features a graph by John Hopkins University ( archive.vn/xZMuz ) that shows the total daily number of virus tests conducted in the U.S. and how many of these tests were positive.

The rate of positive tests actually provides an insight into how much testing is being conducted and what kind of patients are being included in the testing. John Hopkins University explains that a high positivity rate suggests that tests are largely being conducted to the “sickest patients”, possibly meaning that milder or asymptomatic cases are not being tested. A lower rate, on the contrary, may indicate that testing is also including these “milder” patients. ( here )

This question of whether positivity rates are an indication of herd immunity was addressed by a team of global health scientists and infection preventionists at the Meedan Digital Health Lab, a public health information hub here .

The experts note two elements to disprove this claim: first, evidence to support COVID-19 long-term immunity is lacking. “Herd immunity would require a large majority of the population to become infected with the virus and obtain long-term immunity to COVID-19 — but since we know so little about long-term immunity right now, we can’t say anything about herd immunity in relation to COVID-19.”

They also mention that positivity rates can mean “a wide variety of things”. It could mean an increasing spread of the disease if “the rate of positive tests increases while the amount of testing stays the same”. It could also indicate that not enough tests are being conducted, if “more tests come back with positive results but tests were conducted on a smaller percentage of the population than the week before,” for example.

In an op-ed for the New York Times here , Yale immunobiology professors Akiko Iwasaki and Ruslan Medzhitov wrote: “Given the severe consequences of COVID-19 for many older patients, as well as the disease’s unpredictable course and consequences for the young, the only safe way to achieve herd immunity is through vaccination.”

On June 29, Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that even with a vaccine that was 70-75% effective, the U.S. would “unlikely” achieve herd immunity if, as one opinion poll suggested, up to a third of Americans are not willing to get vaccinated ( here ) .

VERDICT

False. The rate of positive COVID-19 tests in the United States does not indicate that herd immunity has been reached.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts  here  . 

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