RALEIGH, N.C. (WNCN) — You rip open your rapid COVID-19 test, swab your nose, dunk the stick into the tube, blot the mixture onto the test strip and wait.
If the line that indicates a positive shows up, it’s becoming more likely that it will be thin, hard to make out and take a long time to finally appear.
The difficulty in reading the result makes for a short-term problem — but it’s actually a good thing in the long term.
Because in the past you’ve been vaccinated, caught COVID — or both — your immune system now is doing a better job of responding.
“This is one of those things that’s a good-news, bad-news kind of place we find ourselves,” said Dr. Thomas Denny, a COVID testing expert and the chief operating officer of the Duke Human Vaccine Institute.
“What we’re seeing here now is a lot of people have had multiple vaccinations and a lot of people have been infected,” he added. “So all that gets us to that point where we were hoping to be one day — a lot of people have antibody levels circulating in their blood.”
For rapid antigen tests, there’s a rule of thumb: The bolder and darker the positive line is on your test strip, the more virus you probably have in your body.
If the line is dim, one of two things is happening, Denny says: Either the viral levels are low, or they are being locked up by the antibodies the person being tested has.
“When the virus hits, that’s what we call the antigen, in this case, and when you have all that antibody circulating in the blood, it can bind together like a lock and key when that happens,” Denny said. “The antigen assays as we know them don’t perform as well. That’s why you’re seeing these dim patterns now.”
Those kinds of positives tend to be, well, more positive than the obvious ones.
“How come it takes a while for my rapid test to become positive?” asked Dr. David Wohl, an infectious disease expert at the University of North Carolina School of Medicine.
“How come my rapid test was only a faint line? How come I only got minimal symptoms, my PCR was positive but my antigen test was negative?” he continued. “This is showing that the virus gets into our system, but then we beat it back.”
What does it mean moving forward?
“We have to be cautiously optimistic here, but I think what’s encouraging here is that people are getting vaccinated,” Denny said. “We always worried about what level of circulating antibodies remained in people after six months of vaccination or nine months and all the subsequent boosters. But if you’re having significant amount of antibody in your circulation, that’s caused a sort of an anomaly, if you will, or a change with the test performance. That’s a good thing because that means that you have that antibody to help fight off the virus.”
You also might have to keep looking closely at your antigen test for that faint line — unless the tests are recalibrated to give more definitive answers to those lower viral levels, Denny said.
“Or, we may just have to live with the fact and educate people that if you’ve been vaccinated a lot or had COVID a couple times and you do an antigen assay, you see a thin line and the control works, then you probably are infected,” he said.
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.