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SARS-CoV-2 Serology Testing Ramping Up to Address Next Stages of Pandemic

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NEW YORK – A number of academic and commercial labs are ramping up development of serology tests for SARS-CoV-2, which could help manage the pandemic as it moves past its initial phases.

Serology tests look for the presence of host antibodies against disease, which can indicate whether a person has been exposed to an infectious agent and potentially whether they have developed a measure of immunity against it.

In the case of SARS-CoV-2, such tests could help identify people who have had the virus and recovered from it, allowing them to safely return to work. Additionally, they could provide researchers with information on the spread and scope of infections. While much of the initial effort around testing has focused on scaling PCR-based testing for the virus, serology tests will likely play a larger role in managing the virus in coming months.

Serology tests may also be useful in development of treatments for the disease. Specifically, doctors are exploring whether transfusion of blood from patients who have recovered from COVID-19 could be effective for treating patients with the virus. Serology testing is necessary to determine that blood from the recovered patients possess anti-SARS-CoV-2 antibodies.

Presumably anticipating the need for these tests, the US Food and Drug Administration announced on March 16, 2020 that it would be substantially loosening its regulations for SARS-CoV-2 serology tests, noting that it "does not intend to object to the development and distribution by commercial manufacturers or development and use by laboratories of serology tests to identify antibodies to SARS-CoV-2," provided that test makers have validated their assay, notified the FDA of the assay, and include with the assay notice that the test has not been reviewed by the FDA, that false negatives and false positives are possible, and that the results should not be used alone to determine infection status.

Test developers who intend for their serology assays to be used alone to determine infection status are still required to take their tests through the FDA's Emergency Use Authorization process.

Serology tests work by using viral proteins or portions of viral proteins to detect anti-viral antibodies in a person's blood, the idea being that the presence of such antibodies indicates that they have been exposed to the virus and generated an immune response to it. These tests can be run in conventional immunoassay formats like ELISA and are fast and relatively inexpensive. They can also be packaged for point-of-care or even at-home use.

The basic process underlying development of a serology test is straightforward — developers identify proteins or protein fragments specific to the target virus and generate them via a recombinant protein expression system. In practice, though, it can be somewhat more complicated as the ideal viral proteins from a specificity standpoint might not be the best proteins in terms of generating an immune response or might not be the easiest to express.

In the case of SARS-CoV-2, the question of specificity is a challenging one given that there are several other common coronaviruses to which many people probably already have antibodies, said Robert Garry, professor of microbiology at the Tulane University School of Medicine. Garry and his colleagues are developing a serology test for SARS-CoV-2.

"That's the major problem," he said. "There are four other coronaviruses around the world that cause mild respiratory illnesses. They come as seasonal colds, and many people are infected with them. So, the question is can you design a test that will only recognize SARS-CoV-2?"

While in theory test developers can optimize for specificity by selecting portions of SARS-CoV-2 proteins that are highly variable across the different coronaviruses, these portions might not work well in recombination expression systems or might not be the portion of the protein that gets exposed to the body's immune system.

The extent to which these challenges will manifest with SARS-CoV-2 remains to be seen, Garry said. "We could get lucky and pick some of the right [proteins], but that is going to take a whole lot more testing."

Last week, a team led by Florian Krammer, professor of microbiology at the Icahn School of Medicine at Mount Sinai, published details of a SARS-CoV-2 serology test it had developed and validated using samples from COVID-19 patients and negative controls collected before the appearance of the virus. The lab has provided instructions for making the viral proteins used in the assay and is also providing the plasmids used for expressing the proteins.

Meagan McMahon, assistant professor of microbiology at Mount Sinai, said that the group shipped out plasmids to more than 30 labs on Monday. On Wednesday, Krammer announced on Twitter that the protein expression plasmids used in the assay are now available from BEI Resources and that interested labs should contact the company to obtain them as his lab was overwhelmed with shipping requests.

The Mount Sinai team used 59 banked serum samples from previous clinical study participants with ages ranging from 20 years to 65-plus. The samples were taken before SARS-CoV-2 appeared in the US, meaning that they were naïve to that virus but likely had exposure to other common coronaviruses. These samples produced only low, near-baseline signal when tested, indicating that the test is specific for SARS-CoV-2. They also tested it in samples from four COVID-19 patients that were obtained at day 20, 6, 4, and 2 after the onset of symptoms and found that the test was effective in detecting immune response in these patients.

McMahon said she and her colleagues have high confidence in the specificity of the assay given the results in the negative control samples but that they are less certain about its sensitivity.

"We haven't looked into the sensitivity too much," she said, adding that this was something they were investigating more moving forward.

McMahon said the assay has now been transferred to Mount Sinai's clinical labs which are working to validate it in that setting. She said the test would be done in two steps — an initial assay to screen for exposed patients with positives from that assay then tested against a second SARS-CoV-2 protein to confirm the initial test result.

In addition to academic labs like those at Mount Sinai and Tulane, a number of commercial companies are developing serology tests. Quest Diagnostics has announced it is working on a serology test, though Steve Rusckowski, the company's CEO, chairman, and president, said the company could not predict how long it would take to bring the test to market. Opko Health's BioReference Laboratories is also developing a serology test, though the company declined to comment on how the effort was progressing.

The FDA currently lists 23 companies that are offering serology tests developed under the agency's new relaxed regulatory guidelines, among them several US Dx firms including Phamatech, Nirmidas, and BioMedomics.

San Diego-based Genalyte is currently prepping a serology SARS-CoV-2 test for use on its FDA-approved Maverick platform. The company has in recent years focused on developing assays for rheumatology applications, most of which involve measuring patient antibody levels. This made SARS-CoV-2 serology testing an obvious fit for the firm's technology, said CEO Cary Gunn.

Gunn said the Maverick platform can look at IgG and IgM responses to 16 proteins simultaneously and return results in about 15 minutes. The company plans to use this multiplexing capability to include proteins from other coronaviruses, which he said would allow it to better control for false positives than conventional serology tests.

He said the company can currently run around 6,000 tests per day out of its CLIA facility and aims to double that over the next six weeks. He said it was also looking to place instruments with large medical centers that could use the test to screen healthcare providers to identify those showing an immune response to the virus, which might indicate they could work with less danger of becoming infected. He said that each instrument could run around 300 samples per day.

Gunn said Genalyte expects to begin offering the test for clinical use in about two weeks.

Tulane’s Garry said that while the relaxed FDA requirements would help speed tests to market, it also raises the possibility that inaccurate tests would make their way into the clinic.

"The cross-reactivity issue is going to be a major one," he said.

Garry cited the UK government's purchase of 3.5 million at-home SARS-CoV-2 serology tests. (At-home tests do not fall under the loosened FDA regulations on serology testing.)

"I understand the UK is buying up a bunch of home-use tests, which, you know, maybe they are great tests. I hope they are. We'll see," he said.

But, there are reasons for concern. A report published today in the Spanish newspaper El País said that 340,000 SARS-CoV-2 serology tests Spain purchased from Chinese firm Bioeasy had a sensitivity of only 30 percent, leading the government to withdraw the test from use.

Another potential challenge for serology test developers are the sort of reagent shortages that have hindered the ramp-up of PCR test production. Garry said that while this has not been at the forefront of his mind, it could potentially prove an issue for serology test development.

"Most of the [reagents] are things you would make yourself if you are developing the test, but there are some common things that I could see going into shortage," he said, citing as an example the anti-human antibodies used in such assays.