RALEIGH, N.C. (WNCN) — How do you know if your COVID-19 vaccine is still working?

Doctors at Duke-NUS Medical School in Singapore are working on a blood test that can give a clearer answer to what’s been a difficult question for the past 1 1/2 years, with a study outlining the test published earlier this month.

There’s a rule of thumb is that immunity starts to fade after 3-4 months, but that’s only based on how part of how the immune system — antibodies — works.

The new test measures another part: The T-cells.

“Basically, it’s a generalized tool that allows us to rapidly test and understand the T-cell response,” Anthony Tan, one of the doctors from Duke-NUS, told CBS 17 News.

T-cells are a big deal because they work differently than antibodies do. Antibodies can leave your body after an infection, but T-cells tend to stick around longer — making them a better way to track long-term immunity.

“The T-cells recognize infected cells, and antibodies recognize a virus, to be very simple,” said Nina LeBert, another of the doctors from Duke-NUS Singapore who is working on the test. “And then basically, the T-cell can kill the infected cell.”

Here’s how the test works: The subject’s blood is mixed with some proteins from the COVID-19 virus, and the mixture is checked 24 hours later for a T-cell response.

“The message is, really, you can now measure the T-cell response easily. You can understand whether you have, and what kind of, levels of T-cell response you have.”

The doctors say it can help immunocompromised people develop a clearer picture of how protected they are from the virus.

“That’s, I think, a very good social benefit of understanding what the T-cell response is,” Tan said.

But there’s an easy way to misinterpret what the test tells you, the doctors say.

“For the general public, I feel that it’s very easy for them to go down the rabbit hole and say, ‘Oh, is this amount (of T-cell response) enough?” Tan said. “And I think that question is the age-old question that nobody knows.”

Tan says when you look at your test results, it’s better to pay attention to where you fall in the range of scores as opposed to the absolute number.

“There’s nobody who knows how much is enough,” Tan continued. “It is just a reference point where you can say, ‘Maybe you need to watch out,’ or ‘You’re completely normal,’ which I think is very important when we are dealing with, for example, immunocompromised people.”

CBS 17’s Joedy McCreary has been tracking COVID-19 figures since March 2020, compiling data from federal, state, and local sources to deliver a clear snapshot of what the coronavirus situation looks like now and what it could look like in the future.