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A new crop of COVID-19 antibody tests measures the level of protection someone has built up against the deadly virus and may help determine which vaccines are most effective.

Siemens Healthineers AG is the first big company to gain U.S. Food and Drug Administration clearance for a test that gauges the concentration of long-lasting antibodies flowing through a person’s blood. Swiss giant Roche Holding AG is following up with its own version.

Antibodies are markers of infection, and testing for them helps health officials see how widely the virus has spread. Until now, most tests could tell only whether they’re present. The newer ones go a step further, measuring the quantity of these proteins, which are raised by the immune system to disable viral invaders.

The level of the body’s response is important in determining whether patients will develop immunity. It’s also key to determining the effectiveness of vaccines as developers bring promising candidates into late-stage trials.

“You need a test that shows whether a vaccine has triggered the right level of antibodies in a patient’s blood,” said Deepak Nath, president of laboratory diagnostics at Healthineers. “It’s important to say how much of it you have.”

Moderna Inc. used a form of quantitative testing to show that all 45 participants in a Phase I trial of its experimental coronavirus vaccine developed antibodies. The company’s Phase III trial, however, is considerably larger — with about 30,000 people. And that’s just one candidate out of a field of more than 160 different shots in development globally.

Taken together, this boom in vaccine research could create a huge demand for quantitative antibody testing — and that need could grow once vaccines actually win approval, since officials will need to monitor their performance as potentially billions of people receive shots. Big diagnostics companies say they’re ready.

“We’ve been talking to many of the vaccine companies and they’re very interested,” Thomas Schinecker, Roche Holding AG’s head of diagnostics, said on a call with reporters last month. Roche is in the final stages of development for its quantitative test and expects to share more information in the coming months, the company said.

It’s been a long and bumpy road for COVID-19 antibody testing. Early on, the main objective of test-makers was to introduce a tool that could simply detect whether someone’s immune system has fought off the coronavirus, to complement other tests that show whether they’re currently infected.

Designing a device to answer that question proved challenging enough, with many countries stocking up on tests that proved unreliable. Eventually, big players including Siemens Healthineers, Roche and Abbott Laboratories brought forth reliable diagnostics to better understand the spread of the virus.

Officials used such tests to estimate that roughly a quarter of New York City residents had been infected by April while about 7.1 percent of people in the U.K. had been as of July.

Until now, antibody tests have been typically looked at for evidence of a range of antibody types, including generally fast-appearing but short-lived ones and another kind that shows up later but provides longer protection.

Healthineers’ new test zeroes in on the latter. Once an organization like the International Federation of Clinical Chemistry and Laboratory Medicine determines what level of these antibodies is needed to confer immunity, researchers will be able to see who needs a vaccine, a booster shot or nothing at all.

Antibodies aren’t the body’s only defense. T cells can kill virus-infected cells, while memory cells linger for years, ready to re-populate the body with both weapons if the virus returns. Still, antibodies are the easiest to measure, making the new quantitative tests a potentially powerful way to better grasp the immune system’s full response.

“What does the antibody mean in contrast to, say, the T-cell mediated response?” Nath said. “From a research standpoint, this is the first tool of its kind to be able to do those studies.”

Roche has another reason to bring forth a quantitative device. The first antibody test it introduced in May detects the presence of antibodies targeting the nucleocapsid that surrounds the coronavirus’s nucleic acids.

However, research has shown that the most protective antibodies are those that target the spike protein that enables the virus to enter cells. Most vaccine developers are coming up with shots inducing the body to make spike protein-related antibodies, potentially weakening demand for Roche’s nucleocapsid product.

Roche’s quantitative test focuses on spike-protein antibodies. The company will then be able to use both of its antibody tests together, a potential benefit since a small percentage of people develop antibodies against either the spike or the nucleocapsid, not both. “You’ll get an even better understanding of what’s happening out there if you do a combination of the two,” Schinecker said.

Quantitative antibody testing could also help with treatments involving convalescent plasma, in which people who’ve already defeated COVID-19 donate their antibody-rich blood to those who are still fighting the virus.

“If five people showed up in a clinic, you can take blood and say, ‘Hey, this person has more antibody versus this other person and therefore is perhaps a better candidate for blood donation,’” Nath said.

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