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This illustration, created at the Centers for Disease Control and Prevention (CDC), reveals the ultrastructural morphology exhibited by coronaviruses. Note the spikes that adorn the outer surface of the virus, which give the appearance of a crown surrounding the virion, when viewed under an electron microscope. With this in mind, the protein particles E, S and M, also located on the outer surface of the particle, were all also labeled. A new coronavirus, called Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), was identified as the cause of an outbreak of respiratory disease first detected in Wuhan, China, in 2019. The disease caused by this virus was named coronavirus disease 2019 (COVID-19).
The world has come to depend on antibody tests to see if someone has ever had Covid-19, but a study in the city where the virus first spread in Italy shows that these tests are not not completely reliable.
A different test – one that looks for an immune cell, called a T cell – was more effective, according to the study.
“It makes sense. It is well known that antibodies decrease, but T cells have immunological memory,” said Dr Peter Hotez, infectious disease specialist at Baylor College of Medicine.
In the study, researchers in Italy, the UK, and the US studied people in Vo, Italy, to learn more about test accuracy.
They performed blood tests on 70 people who had had confirmed cases of the coronavirus about two months earlier.
Theoretically, all 70 of them should have had positive results on an antibody test. But the antibody test gave negative results in 16 of the cases, or 23%. The T cell test only missed 2 cases, or about 3%.
Researchers also tested 2,200 people who had tested negative for Covid-19. Of these, the T cell test returned positive results for 45 of them.
A co-author of the journal said he suspected that many of those 45 people had had coronavirus at some point, but did not realize it. Of those 45 people, 25 had symptoms of the disease at some point or had lived with someone who had had a confirmed case of Covid-19, or both.
This co-author, Dr. Lance Baldo, is the chief medical officer of the company performing the T cell test used in the study.
The data was announced Tuesday during an investor call for Adaptive Biotechnologies, the company doing the test. The company did not fund the study and the data has not been released.
Adaptive plans to launch its T cell test on the market in late November and plans to seek emergency use authorization from the United States Food and Drug Administration.
While there are currently T cell tests used in research, they are not widely used commercially for patients.
Other studies, like a large study carried out last summer in the UK, have also shown that antibodies decrease over time.
There is more than one way to theoretically test for a previous infection with a virus, as many different cells – including T cells and antibodies – are involved in the body’s efforts to fight the virus.
“It’s like a military operation, where you have different components. The navy lands on the shore, the air force attacks from above, the army comes in with artillery,” said Dr Andrew. von Eschenbach, former commissioner of the FDA. “When something tries to take hold of us, the fight our body engages in is extremely sophisticated and complicated.”
Once the body has fought off the invader, the antibodies usually decrease over time.
“The source of the antibodies – the factory that makes them – goes out in a matter of months,” said Akiko Iwasaki, an immunologist at the Yale School of Medicine.
On the other hand, T cells stay for a while.
“For other viruses, they have been shown to persist for years. For this virus, we don’t know how long they last, but we would expect at least a few years, ”she says.
Not only do the T cells stay, but they remember how to specifically fight the coronavirus.
“You make a lot of them, and they live in lung tissue and other tissue affected by the virus – they’re just sitting right there,” Iwasaki said.
A study in the UK earlier this month showed that functional T cell responses to Covid-19 infection were retained for six months after infection. It was posted on a pre-print server but has not been reviewed or published in a medical journal.
Even then, it is not known if the T cells will protect someone from Covid-19 a second time.
“This is an experiment that Mother Nature is going to conduct for us,” said von Eschenbach.
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