(CNN)The world has come to depend on antibody tests to check whether someone's already had Covid-19, but a study in the town where the virus first spread in Italy shows that these tests are not completely reliable.
A different test -- one that looks for an immune cell, called a T cell -- was more effective, according to the study.
"This makes sense. It's well known that antibodies wane, but T cells have immunological memory," said Dr. Peter Hotez, an infectious disease specialist at Baylor College of Medicine.
In the study, researchers in Italy, the United Kingdom and the United States studied people in Vo, Italy, to learn more about testing accuracy.
They conducted blood tests on 70 people who had had confirmed cases of coronavirus about two months earlier.
Theoretically, all 70 of them should have had positive results on an antibody test. But the antibody test returned negative results in 16 of the cases, or 23%. The T cell test missed only 2 cases, or about 3%.
The researchers also tested 2,200 people who had tested negative for Covid-19. Of those, the T cell test returned positive results for 45 of them.
A coauthor on the paper said he suspects that many of these 45 people had had coronavirus at some point but didn't realize it. Of those 45, 25 either had symptoms of the disease at some point or had lived with someone who had had a confirmed case of Covid-19, or both.
That coauthor, Dr. Lance Baldo, is the chief medical officer of the company that makes the T cell test used in the study.
The data was announced Tuesday at an investors' call for Adaptive Biotechnologies, the company that makes the test. The company did not fund the study and the data has not been published.
Adaptive is planning to launch its T cell test commercially in late November and plans to apply for an Emergency Use Authorization from the US Food and Drug Administration.
While there are currently T cell tests used in research, they are not in wide commercial use for patients.
Other studies, such as a large one last summer in the UK, have also shown that antibodies wane over time.
There's more than one way to theoretically test for prior infection with any virus because so many different cells -- including T cells and antibodies -- are involved in the body's effort to fight off the virus.
"It's like a military operation, where you have different components. The Navy lands on the shore, the Air Force attacks from on high, the Army comes in with artillery," said Dr. Andrew von Eschenbach, a former FDA commissioner. "When something tries to invade us, the fight our body launches is extremely sophisticated and complicated."
Once the body has fought off the invader, antibodies will usually wane in time.
"The source of the antibodies -- the factory that makes them -- dies off within a few months," said Akiko Iwasaki, an immunologist at the Yale School of Medicine.
On the other hand, T cells stick around for a while.
"For other viruses they've been shown to persist for years. For this virus we don't know how long they last, but we would expect for a couple of years at least," she said.
Not only do T cells stick around, but they remember how to fight off coronavirus specifically.
"You make a lot of them, and they live in lung tissue and other tissue effected 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 published in a pre-print server but has not been reviewed or published in a medical journal.
Even so, it's unclear if T cells will protect someone from getting Covid-19 a second time.
"That's an experiment Mother Nature will carry out for us," von Eschenbach said.
