As states gradually loosen stay-at-home orders, public health officials caution that widespread testing will be critical to safely relaxing social distancing guidelines. Unfortunately, this has been difficult to achieve with the tests available at present. Dr. Deborah Birx, the White House coronavirus response coordinator, said that antigen tests might be “the breakthrough innovation in testing” that the public needs.
Currently, the most common coronavirus tests are polymerase chain reaction (PCR) tests, which detect the virus’ RNA. A health care worker swabs a patient’s nose or throat, and the sample is sent to a lab, where it is mixed with certain chemical reagents to replicate the RNA before the sample is analyzed for genetic materials. PCR tests are the most accurate tests available, but there have been numerous obstacles that limit its efficiency.
When the C.D.C. released its first batch of PCR tests early on in the pandemic, many public health labs reported the tests were faulty and provided inconclusive results. Fixing this error resulted in a three-week delay in the national testing program, which hampered early response efforts.
A shortage of materials for PCR tests, including swabs and reagents, has created another setback. PCR tests are difficult to make and require special equipment and training for lab technicians, which makes them relatively expensive. They also take hours to process, which makes the ordeal too time-consuming for widespread testing. "There will never be the ability on a nucleic acid test to do 300 million tests a day or to test everybody before they go to work or to school," Dr. Birx said earlier this month.
Antigen tests, according to Dr. Birx, are the answer. Antigen tests look for special proteins on the surface of the virus, and are often used for the flu and strep. The C.D.C. said that antigen tests could be completed in as little as 15 minutes, significantly cutting down the wait time.
An antigen is a foreign substance that the immune system detects and reacts to by releasing antibodies, proteins that fight antigens. Each virus has a unique structure, like human fingerprints. Antigen tests work by identifying these structures.
The antigen test must be able to consistently detect the proteins on the coronavirus. Creating an accurate antigen test, however, is difficult because it must be carefully attuned to the structure of the virus. "Sometimes viruses may have special folds, or protein modifications on their surface, which can interfere with the process," said Gronvall.
Additionally, antigen tests cannot detect small amounts of the virus because it doesn’t replicate the genetic material. PCR tests amplify the RNA that is present, so it works even if there are only small amounts of the virus.
Still, antigen tests— when they work— can save a lot of time and money. The tests don’t require special equipment, and return results quickly. This means they could be mass produced and distributed to the public for home use. The tests would have a strip for samples from the patient, which would give an indication if they had the virus, similar to a pregnancy test.
The federal government has funded research into an antigen test for COVID-19. OraSure, the company that developed an antigen test for Ebola, was granted $710,000 by the Biomedical Advanced Research and Development Authority (BARDA), under the US Department of Health and Human Services, to advance work on an antigen test for the novel coronavirus.
The antigen test would use saliva samples and could theoretically return results within minutes. Steven Tang, CEO of Orasure, is aiming for releasing the test by September. Tang plans to ship millions of tests upon release.
BARDA also rewarded Nanomix, a different company, with $500,000 to develop an antigen test for the novel coronavirus. Other companies have received funding for testing as well, but are focusing on other types of tests.
Despite the efforts by companies and the federal government to develop an antigen test for the novel coronavirus, the WHO urges caution regarding the tests until there is more research. The WHO cited mixed results from the antigen tests’ ability to accurately detect the coronavirus, which ranged from 34%-80%.
"Based on this information, half or more of COVID-19 infected patients might be missed by such tests, depending on the group of patients tested," the WHO said. The antigen tests need to be more accurate to be effective. The tests have returned false positives as well, because it detected a different coronavirus, such as the common cold.
The WHO said that antigen tests could “rapidly identify patients who are very likely to have COVID-19, reducing or eliminating the need for expensive molecular confirmatory testing," if more accurate tests are developed.
The antigen tests are not expected to replace PCR tests, but they will increase the scale of testing in the US, a necessary measure as social distancing guidelines are gradually relaxed. Patients who test positive using an antigen test can confirm the result with a PCR test. Widespread testing will make contact tracing possible, and those without the virus can return to work. As the public returns to work and school, antigen tests may be the key to easing stay-at-home orders safely and efficiently.
[Sources: Madison.com; CNN; New York Times; NBC]