For much of the pandemic, the keen sensitivity of the primary test for COVID-19 has been a blessing, detecting even small amounts of the virus that can spread silently among people who might not feel sick.
But as more people get vaccinated, the test’s sensitivity is raising questions among health experts about whether results should be looked at more critically when an immunized person with no symptoms tests positive for COVID-19. Is that person truly a threat to spread the virus, or just fighting it off and keeping it at levels too low to be contagious?
Kevin Miller got caught in the middle of that unfortunate debate and it cost him some of the most precious moments of his high school senior year.
The Los Gatos High School student had jumped at the opportunity to be vaccinated, getting his final dose Feb. 27 so he could return to class on campus and play basketball on the varsity team as soon as possible without worrying about the virus. But May 5, almost 10 weeks later, what seemed impossible happened. A test, required before his games, came back positive.
“I had no reason to believe that I had it,” said Miller, 18. “I never felt sick, never got another positive test. No one else on the team tested positive.”
Despite three subsequent negative tests, local health and school officials said they were forced to implement safety protocols, requiring Miller and dozens of his exposed team members and classmates to quarantine for a week, missing in-person classes and games, and nearly keeping them from attending their senior prom.
“It just felt like this situation where you can’t make these things, up the way this was unfolding,” said Kevin’s mother, Dana Miller.
Some health officials say the ordeal shouldn’t have been necessary. Vaccinated people who aren’t feeling sick might still trigger a positive COVID-19 test while their body fights off the virus, but aren’t at risk of infecting others, said Dr. Monica Gandhi at the University of California-San Francisco, who has studied virus safety in schools.
“I think we need to change our definition of what is an infection at this point,” Gandhi said.
It’s a scenario likely to come up more often as more people are immunized and schools and businesses fully reopen with an eye toward preventing outbreaks. Though the U.S. Centers for Disease Control and Prevention doesn’t recommend COVID-19 testing for vaccinated people without symptoms, there are still situations, like some athletic competitions, where players are screened for the virus.
It wasn’t the first time a fully vaccinated athlete tested positive for COVID-19 without any symptoms of the disease and was forced to quarantine. Around the same time last month, nine fully vaccinated members of the New York Yankees baseball team tested positive for the virus, including star shortstop Gleyber Torres.
Only two reported mild symptoms, while Torres and the rest were asymptomatic. But all were forced to quarantine. Major League Baseball requires two negative COVID-19 tests for the vaccinated to exit a 10-day quarantine early.
The CDC considers any positive test for SARS-CoV-2, the virus that causes the disease, in a person at least two weeks past their final vaccine dose a “breakthrough” infection. A recent CDC study called those “very rare” — just 0.01% of fully vaccinated Americans this year tested positive, 27% of whom showed no symptoms.
How accurate are COVID-19 tests? The polymerase chain reaction, or PCR, tests commonly used to confirm COVID-19 are considered highly accurate, to a point.
If the infection hadn’t progressed far when the person was swabbed, it might not trigger a positive result, so health authorities warn a negative test doesn’t prove a person is COVID-free. They also consider false positives highly unlikely. Unless the sample was contaminated, some virus must be present.
Do the tests indicate the severity of infection — and contagiousness? Many experts have argued it does.
The PCR test analyzes samples by amplifying viral RNA, or genetic material, in typically 40 amplification cycles. If virus is detected in 40 cycles or less, it’s considered positive. The more cycles it takes to detect the virus, the less there is. A test that finds the virus in 20 cycles indicates a far higher viral load in the patient than one that takes 40 cycles. Patients with higher viral loads are more contagious, while low viral loads aren’t necessarily infectious. It isn’t clear what Kevin Miller’s viral load was on his positive test.
Before vaccines, a patient with a lower viral load might be assumed to become more infectious as the illness progresses. But for the vaccinated, a low viral load that triggers a positive test in a person feeling no symptoms may simply indicate the vaccine is doing its job.
“It shows the effectiveness of the vaccine in fighting that virus in your nose,” said Gandhi. She argues that cases like Miller’s or the Yankees were “not a true positive test — that was the vaccine working!”
She isn’t the only health expert calling for health authorities to reconsider how PCR tests are evaluated for the vaccinated. After the Yankees outbreak, Harvard University epidemiologist Michael Mina argued in New York magazine that the breakthrough term should be reserved for vaccinated people who become sick. He noted that effective vaccines generally aren’t “fully sterilizing,” killing all the virus instantly.
But other experts say caution is still warranted.
“I don’t think there is any evidence that vaccinated people with a low viral load are less likely to transmit than unvaccinated people with a low load,” said Marta Maria Gaglia, a professor of molecular biology at Tufts University who has studied how the COVID-19 virus evades human immune responses.
Despite the debate, CDC spokeswoman Martha Sharan said the agency isn’t planning to change what it considers a vaccine breakthrough infection.
“The passive surveillance system we have established is not meant to evaluate contagiousness and the data should not be used in that way,” Sharan said. “Additional laboratory-based studies are needed to understand if breakthrough cases can transmit the virus. Our system is meant to track people who test positive for SARS-CoV-2 post vaccination and track the trends in these cases, not evaluate infectiousness.”