Until recently, it’s been nearly impossible to say. Immunity, whether from vaccine or prior infection, is thought to wane after three or four months, but it varies by person. That knowledge is based on what’s known about typical antibody response—but antibodies are only half of the picture. 

The other half: T-cell response, which hasn’t been examined in patients nearly as often owing to technical challenges. Now that response can be tested affordably and en masse, researchers at Mount Sinai Health System in New York say. 

Along with researchers at Duke-NUS Medical School in Singapore, they developed a rapid blood test called the dqTACT assay that measures the activation of such cells in response to COVID. The test will allow for mass monitoring of the population’s immunity and effectiveness of vaccines new and old, they said in a study published Tuesday in Nature Biotechnology.

Coupled with an antibody test, the rapid T-cell test can provide a complete view of an individual’s immunity, said Jordi Ochando, assistant professor of oncological sciences at the Tisch Cancer Institute at Mount Sinai and a member of the team that developed the test.

Such a view will enable researchers to make “more refined decisions regarding vaccination strategies, especially in high-risk patient groups,” Ochando told Fortune.

For some patients, personal health means public health

The test can also help doctors develop individualized vaccination recommendations for immunocompromised patients.

The test is “very informative” for cancer patients taking B-cell depletion medications like rituximab, or who have genetic defects affecting antibody response, Ochando said. “These patients need to be monitored for T-cell immunity in response to vaccination and/or infection to determine their level of immune protection and act accordingly.” Protecting immunocompromised patients is important—not only for their personal health, but for public health. 

Researchers have witnessed variants evolve in those with long-term infections. The B.1.1.7 wave that shut down the U.K. in late 2020 contained a mutation that was found in an immunocompromised patient who eventually died 101 days after being diagnosed, despite antivirals and two rounds of plasma from recovered patients. That mutation may have allowed the virus to escape the infusions of antibodies he received.

And in a July 2021 study in medical journal mSphere, researchers describe watching the virus evolve in an immunocompromised patient over six months, eventually developing mutations that may have allowed immune evasion and/or enhanced transmission.

Test comes as antibody levels are increasingly irrelevant

The assay—which has been granted certification in Europe and is undergoing clinical validation by the U.S. Food and Drug Administration—comes as subvariants increasingly develop the ability to evade immunity, making antibody levels less relevant.

“The emergence of SARS-CoV-2 variants like Omicron, which evade most of the neutralizing ability of antibodies, points to the need for assays that can measure T cells, which are more effective against emerging variants of concern,” Ernesto Guccione, professor of oncological and pharmacological sciences at the Tisch Cancer Institute at Mount Sinai, said in a statement. 

Levels of Omicron subvariants BA.4 and BA.5, known to evade immunity, are rising in the U.S., with levels of BA.5 giving once-dominant BA.2, also known as “stealth Omicron,” a run for its money, according to data from the U.S. Centers for Disease Control and Prevention released Tuesday.

And levels of the two new immunity-evading variants are “increasing around the world,” said Maria Van Kerkhove, technical lead for COVID-19 response at the World Health Organization, on Tuesday.

A recent study out of South Africa found that those who had been previously infected with Omicron but not vaccinated experienced a nearly eightfold drop in neutralizing antibodies when exposed to BA.4 and BA.5. Those who had been vaccinated and previously infected with Omicron saw a milder threefold decrease. 

It’s dismal news, but T cells provide a ray of hope: An April study published in eBioMedicine found that critically ill COVID patients mounted a T-cell response that was sustained more than a year after infection.

And a 2011 study of SARS coronavirus patients showed that many retained T-cell memory after six years, though they did not have antibodies.

T-cell immunity might outlast antibody immunity—at least in some patients. Soon, you can find out your truth for yourself.