Using rapid antigen tests to screen for COVID-19 in people with no symptoms hasn’t been the pandemic “game-changer” that some experts predicted back in the days before vaccination.

But the practice is growing, boosted by federal pandemic funding and the need to reopen society even as vaccination rates stagnate and case rates rise due to the delta variant.

Cruise lines and many airports around the world are providing, or requiring, simple, cheap, use-anywhere antigen tests to identify people who need to quarantine. So are some college campuses and prisons. Visitors to California hospitals and nursing homes must have proof of vaccination, a negative molecular (PCR) test — or a negative antigen test. Even the Made in America festival, returning to Philadelphia on Labor Day weekend, is offering antigen tests at the gate for people who forgo vaccination or the diagnostic gold-standard PCR test.

Frequent antigen testing is also becoming an important tool in K-12 schools. Public health experts at Children’s Hospital of Philadelphia, for example, are spearheading an antigen testing initiative that is open to all schools in Southeastern Pennsylvania, and is modeled on a similar program in Texas.

Forty-six school districts in the suburban counties and Philadelphia are now participating, and 372,000 antigen tests have been administered since the program started in January.

“While evidence suggests that schools have not typically been a significant source of COVID-19 spread, maintaining a learning environment in which both students and staff feel safe has been a challenge,” says the website of the project, called ACE-IT (Assisting Childhood Education through Increased Testing).

» READ MORE: To reopen faster and more safely, the world needs a COVID-19 test for people with no symptoms. Here are the hurdles.

In some ways, that challenge is unique. Many students aren’t vaccinated, either because they’re under age 12 and ineligible, or the choice rests with their parents. Masks are impractical for many children, such as those with special needs. And for most children, in-person learning is far better than the virtual version for academic and social development.

Antigen testing in schools has not yet been proven to reduce the spread of COVID-19. Data are mixed and scant, especially regarding asymptomatic students and teachers.

But screening advocates point out that antigen testing has clear advantages over expensive, time-consuming PCR. While antigen tests are less accurate than PCR, they can signal a new infection even before it causes symptoms by detecting the viral protein, or antigen, that surges and then plummets early in infection. This early stage is when people are most contagious.

The accuracy debate “is the conversation that has dragged us down throughout the pandemic,” said Richard Pescatore, chief physician with the Division of Public Health in Delaware, which is partnering with Quidel Corp. to provide antigen testing in schools throughout that state. “PCR is overly sensitive for COVID-19″ because it can detect virus that is no longer transmissible.

Atlanta’s antigen experience

Antigen screening in schools is optional, and parents have to give consent for their children. Each participating school tailors testing to its needs, deciding frequency, locations, target groups, implementation, and more. Parents, meanwhile, often need to be persuaded to sign on. They might not see the value, or worry that testing will waste time, or that false results may lead to unnecessary quarantines. (PCR is used to confirm antigen results to avoid that.)

“The success of the program is 100% going to depend on the level of participation,” said Pescatore. “So it’s dependent on marketing and communication efforts.”

The Atlanta school district was in the vanguard of antigen screening, piloting a program last spring and promoting it through open houses, newsletters, fliers in backpacks, the PTA, and more.

This school year — already underway in person for the city’s 52,000 public school students — is showing the value and limits of screening in a district where less than a fifth of eligible students are vaccinated and community spread is surging.

The Drew Charter School, where almost all teachers and students have consented to routine antigen testing, made national headlines when two students and two employees tested positive, leading to the quarantine of more than 100 students. (Those numbers have climbed with ongoing testing.)

Meanwhile, at Atlanta’s Parkside Elementary, participation is so-so.

“We have over 500 students. About 250 have been consented,” Parkside PTA president Darcy Rice Millard said Tuesday. “I really wish it was more.”

Atlanta Public Schools, like CHOP’s ACE-IT and other screening initiatives, are also offering free PCR testing and holding vaccination clinics.

“It’s been nice to see how much APS is doing,” Millard said. “Without outright mandating the vaccine, they’re doing all they can to keep kids and staff safe.”

The difference in COVID-19 tests

COVID-19 testing has come a long way from the start of the pandemic, but confusion over test types and their uses remains widespread.

Molecular, or PCR, tests detect fragments of viral genetic material in respiratory or saliva samples. PCR remains the gold standard for diagnosis because it is the most accurate. Many of the several-hundred authorized PCR tests now require just a home-collected, front-of-the-nostrils swab (not a deep nose dive at a drive-through collection site) that is shipped to a lab for analysis.

Antigen tests can signal a new infection even before it causes symptoms – and when it is most transmissible -- by detecting a protein, or antigen, the virus starts making soon after it invades the body. A nasal swab or saliva sample is placed directly into chemicals that break open and react with the virus if it is present, giving results in as little as 15 minutes.

Antibody tests use a blood sample to look for disease-fighting proteins called antibodies, a sign of immune response to infection. In theory, the tests could identify people with immunity who could safely go back to work. In reality, test results are often wrong, and the strength of natural immunity remains uncertain. Antibody tests have become important tools for studying that question, and for understanding the scope of coronavirus exposure in populations.