How protected are we against Covid-19? Scientists search for a test to measure
“I am on immunosuppressants that make me more vulnerable for serious Covid complications,” said Sobieck, 37, who lives in Minnesota. “If you’re immunocompromised, you may not have a very good response to the [Covid-19] vaccine.”
Seeking evidence that his immune system was working the way it should, Sobieck made an unusual request: He asked his nephrologist to do a blood test that gives a rough measure of antibodies, a type of protein the body creates in response to an infection or vaccine. Antibody titers reveal the concentration of a specific antibody found in someone’s blood.
Millions of Americans — not just those with weakened immune systems — are wondering about their protection after a winter of booster shots and Omicron infections. As mask mandates are lifted and restrictions removed in a step toward normalcy, a test to measure immunity would be a powerful tool to measure individual risk.
“The biggest reason I wanted to get my antibody titer checked is because I don’t know how to assess risk,” Sobieck said. “Anyone who is immunocompromised, from the beginning of this pandemic right through today, has very few tools to assess risk: if they’re going to leave the house, when they’re going to leave the house, how to interact with other people, which situations are OK.”
Sobieck had his antibody levels checked after his second, third and fourth doses of the Moderna Covid-19 vaccine. Each result showed that he had more than the maximum number of antibodies the test could detect, indicating a robust immune response.
Although there is no specific guidance on how to interpret these results to figure out the level of protection against infection or disease, Sobieck felt reassured that his immune system was doing its job.
“More than 50% of transplant patients don’t have enough immune response to be protected, even if they get not two but three doses of the vaccine that we use in the general population,” said Dr. Dorry Segev, professor of surgery at New York University Langone Health. “For them, [an antibody titer] is a particularly poignant indicator of whether they have any protection at all.”
Segev, a transplant surgeon, advocates using tests that measure antibodies as a way to check immune protection in people who are immunocompromised.
Sobieck says the test results allowed him to make decisions for himself and his family.
“Knowing that I had the antibody response that I had meant that my son could go to in-person school. That’s huge,” he said.
Correlates of protection
About 95% of Americans 16 and older have antibodies against Covid-19 as of December, the most recent date that data is available, according to US Centers for Disease Control and Prevention estimates that use information from blood donors.
But it’s one thing to measure antibody levels. It’s another to measure how much they protect you against Covid-19.
The US Federal Drug Administration recommends against checking antibody levels at all because there’s no agreed-upon way to calculate how any given antibody level protects you from infection or severe disease. It may also give a false sense of security, the agency says.
“There aren’t good correlates of protection — something that says that this is the measurement that one needs to know how well they are protected,” Emory University virologist Mehul Suthar said.
Scientists are trying to fill the knowledge gap. Studies measure average antibody levels across a population to check vaccine efficacy, often using antibody levels months after vaccination to determine the need for a booster. One study in people who got the Moderna vaccine found that higher antibody levels after vaccination were associated with lower risk of Covid-19 infection.
But not all antibodies are created the same. Of all the antibodies that the body may make after infection or vaccination, only a fraction are considered “neutralizing antibodies,” meaning they can actively prevent infection.
Tests to measure antibodies can be either quantitative or qualitative. Quantitative tests provide a specific number, up to a certain point, of antibodies in the blood. Qualitative tests will indicate only whether certain antibodies were detected. Results of qualitative tests are either positive, negative or indeterminate for neutralizing antibodies.
When it comes to measuring neutralizing antibodies specifically, there is only one type of test that has been given emergency FDA authorization to detect them, and it is qualitative.
Several studies have demonstrated that neutralizing antibodies are a strong correlate for protection against symptomatic infection with Covid-19 and its variants, with boosters enhancing neutralization.
Peter Gilbert, professor of vaccine and infectious diseases at the Fred Hutchinson Cancer Research Center and lead author of the Moderna study, says these correlates are useful to apply to large groups. For example, scientists can use data from larger studies about the correlation between antibody levels and…
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