A U.K.-based study has found that a small sample of healthcare workers who tested positive for SARS-CoV-2, the virus that causes COVID-19, were still showing signs of immunity six months after infection.

Researchers from the UK Coronavirus Immunology Consortium, Public Health England and Manchester University NHS Foundation Trust released the preprinted information Nov. 2. The groups collected serum and blood samples from 2,000 healthcare workers, of whom 100 had tested positive for SARS-CoV-2 in March or April. Blood samples were collected from these 100 workers after six months and all were found to have continued T cell responses.

T cells are a part of the adaptive immune system that begin developing soon after infection and long before the appearance of antibodies, which are produced by B cells. Some T cells, called "helper" T cells, coordinate the body's adaptive immune response, while "killer" T cells can kill infected cells.

Because of their fast production and ability to kill infected cells, some people with milder forms of COVID-19 may not even produce antibodies because their T cells will take care of the virus, according to Adaptive Biotechnologies Corp. President Julie Rubinstein.

"Everybody's immune system, we like to say, has its own story to tell. And we're seeing that play out in real-time with COVID," Rubinstein told S&P Global Market Intelligence in an interview.

The Seattle-based biotechnology company is particularly interested in T cells and the role they play in helping patients recover from a number of illnesses. Over the past few months, the company has been working with Microsoft Corp. on a project called ImmuneCode to better understand COVID-19. The project's goal is to map out the thousands of T cell receptors that reacted with the SARS-CoV-2 virus in order to help guide research into testing and treating people. As of Oct. 19, the companies have successfully mapped out 135,000 T cell receptors to different parts of the virus and have made that data publicly available to researchers.

"If you get the virus, let's say on Monday, you'll start having T cells probably by Tuesday," Rubinstein said. "When the right T cell finds its match to the virus, it makes copies of itself to strengthen the immune response that it's going to coordinate."

Adaptive is also using the results of its ImmuneCode study and other technology to create a diagnostic test that can determine a past infection of COVID-19.

Rubinstein said Adaptive's researchers have also found that T cells persist in the body for a long time, so a T cell diagnostic could prove better than current serology tests in detecting past COVID-19 infections.

In a preprinted study, researchers tested Adaptive's immunoSEQ assay against two serology tests: Roche Holding AG's Elecsys Anti-SARS-CoV-2 test and Laboratory Corp. of America Holdings' SARS-CoV-2 Antibody, IgG test. In 100 subjects who reported having had a COVID-19 infection, the T cell classifier flagged 94 of them, while the Roche and LabCorp antibody tests only reported 90 and 87 prior infections, respectively.

Rubinstein said the extra sensitivity of the T cell-based test will be especially helpful for COVID-19 vaccine-makers who need to confirm that clinical trial participants have not previously been infected and, later, whether participants still came down with the virus after receiving either the vaccine or placebo.

Adaptive is in talks with the U.S. Food and Drug Administration over filing an emergency use authorization for the T cell-based diagnostic. Rubinstein hopes that the company will be able to file the application later this year. The ultimate goal, however, is to use this technology to create diagnostics for many other illnesses as well.