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As more people are considering COVID-19 booster shots, some to help guide their decisions -- but many don't know what to make of the values on their readouts.

Even in the context of reference ranges, there's still one huge problem: there's no definitive titer that guarantees protection just yet.

"The real answer is, we just don't know" that threshold, Nathaniel (Ned) Landau, PhD, a virologist in the department of microbiology at NYU Grossman School of Medicine, told MedPage Today. "It takes time to gather that data, to know what titers people have and what their chance is of getting infected."

Still, researchers are getting closer to that gold standard, and some have offered general context as to what levels may be protective based on the early literature and their own research.

Neutralization Assays: the Gold Standard

An important point to remember is that most commercially available tests -- delivered at the point of care in a clinician's office or performed in a hospital lab -- aren't neutralizing assays, said Clayton Wilburn, MD, of the College of American Pathologists' Clinical Chemistry Committee.

"Antibodies can be detected by those tests, but it doesn't mean they have good activity against the virus," Wilburn, of the University of Vermont Medical Center, told MedPage Today.

The neutralization assay is the gold standard, the researchers said. In these tests, serum from an infected or vaccinated person is diluted to varying levels, then mixed with a set amount of virus. A dilution of 1:100, for instance, means that 50% of virus was still killed when 1 mL of serum was mixed with 99 mL of saline, Landau said.

The more the serum can be diluted and still kill 50% of virus, the better, he explained.

So where do COVID antibodies fall on that scale, at this very early stage in the game?

Based on the literature and on his own research, Landau said natural infection with wild-type virus generally confers a titer of 1:400. With Delta, that gets bumped down to 1:100 -- which is still pretty good, he noted.

Immunity conferred by the mRNA vaccines, on the other hand, typically lands at a titer of 1:1,000 when looking at the original virus, he added.

"It's a lot better," Landau said of vaccinated immunity for the mRNA products. "We don't totally understand why they work so well, but they do."

"One thing," he noted, "is that viruses are really good at messing up the immune system. If you get infected, your antibody production isn't that [streamlined]. But when you get an mRNA vaccine, you don't have all those various parts of the immune response."

He said that titers for the Johnson & Johnson vaccine, at this point, don't appear to be as high as the mRNA products. And protection was certainly diminished for the mRNA products when tested against the Delta variant, he added.

With Delta, mRNA vaccinated immunity falls to a titer of about 1:250.

"We know that [naturally infected] people have been fairly well protected against reinfection ... so that gives you an idea that maybe a titer of 1:100 gives quite good, though not perfect, protection from infection," he said. "So the mRNA vaccines should still give strong protection against variants."

A Different Approach for Commercial Tests

Most of the commercial tests done in doctors' offices and hospitals don't tell us about antibodies' neutralizing capabilities, Landau said.

These typically can tell how much antibody sticks to the spike or nucleocapsid protein, but they don't examine the extra step of killing virus or pseudovirus, he explained.

"Neutralizing antibody tests are more costly and more technically difficult," Landau noted.

"Commercial tests look at overall antibodies that bind to the target, so that can include some with neutralizing capability and some that would not have good neutralizing capability," said Wilburn.

Most of these tests read out positive or negative. While they can provide a numeric value, these values can range from manufacturer to manufacturer. There is no standard across these tests, which are , not approved, experts said. The FDA's most recent update lists a total of .

Still, some researchers have been working closely with those tests for many months now, including Dorry Segev, MD, PhD, of Johns Hopkins Medicine in Baltimore, who has been studying transplant patients -- an immunocompromised population that's been particularly concerned about their levels of protection against COVID.

Segev has published a number of papers on this population, including a of 30 transplant patients who received third doses of COVID vaccines, and has correlated neutralization assays with commercial antibody test readouts. He has now studied some 400 transplant patients who've gotten three doses of the vaccine.

He has primarily worked with LabCorp, which reads out positive or negative for spike IgG antibodies (a test is negative if levels are below 0.8 units/mL; you can see ). The report also provides a numeric value, but only in a generic "units/mL."

Segev said he and his team do see some clear trends in the LabCorp data. They've confirmed that "antibody levels correlate with pseudoviral and live virus neutralization, and the curves are threshold linear."

That means, for LabCorp at least, "until you reach 250 units/mL, you have little evidence of neutralization," he added.

"When you get to 500 or 1,000, it rises in a linear fashion," Segev told MedPage Today. "So 2,000 gives you twice as much neutralizing capability as 1,000 on a LabCorp test."

Segev is to interpret what it can mean, generally, for protection against COVID.

For levels below 250 units/mL, "you have, at most, a modicum of protection," he noted. Those in the 250 to 500 range who are at low risk of exposure -- working from home, taking precautions -- should get a booster, "but there's no urgency." For those in the 500 to 1,000 range who have some risk of exposure or are more vulnerable to COVID, "then you're in the 'now-is-probably-a-good-time-to-get-boosted' range."

"I suspect if you're over 1,000, and not at high risk, then you're probably good," he said.

He cautioned, however, that there's "not a cutoff at which you are protected or not protected."

"Everyone wants a yes or no. Am I safe or unsafe? Protected or unprotected? That's not how it is," he continued. "You're more protected at 2,500 than at 1,000. It's up to you and your risk of exposure, your risk of severe disease, all of those things together, to know whether you need to be at greater than 1,000 or if 1,000 is fine for you."

True Clinical Utility?

Others are less confident about using commercial antibody testing to guide discussions around boosters.

"When people ask me personally or professionally [about getting a booster], I tell them, if you're in a group that's been told it would benefit from the booster, then get a booster," Wilburn told MedPage Today.

He noted that both and recommended against post-vaccination serology.

The University of Vermont Medical Center doesn't offer commercial antibody testing widely to patients because of its lack of clinical utility, he said. Instead, clinicians there can order those tests if they're investigating suspected long COVID or MIS-C.

"We use it in people who have never had a positive PCR test for COVID, but it's suspected that their symptoms are COVID-related," Wilburn explained. "We're trying to learn if they've had natural infection."

That's another caveat of antibody testing. Commercial tests typically look for either spike or nucleocapsid antibodies. Since vaccines create only the spike protein, vaccinated individuals would turn up negative on a nucleocapsid antibody test, essentially giving them a false result. Being positive for nucleocapsid would only indicate previous natural infection.

Cellular immunity -- B cell and T cell counts -- is another part of the picture when it comes to COVID protection, but antibody tests won't reveal any information about an individual's level of protection on these fronts.

Yet ultimately, antibody titers, proven via neutralization assays, will still be the gold standard, just as they are for other infectious diseases.

We're still some time away from having that definitive information, Wilburn warned.

Companies that will want to say that their assays can correlate antibody measurements to neutralization assays will have to get their data together and file with the FDA to be authorized or approved to make those claims, he noted.

Titers for common diseases like measles, mumps, rubella, and whooping cough have "all been studied for multiple years," he said. "Those assays are set back to those gold standards. [With COVID], we're not at that stage yet."

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