Scientists Search for a Test to Measure Immunity Against COVID

Millions of Americans are wondering about how protected they are against COVID-19 after being infected or boosted. Studies have shown that protection wanes over time. Those who are immunocompromised may not have an adequate response from vaccines and could be vulnerable to illness, especially in light of the highly contagious omicron variant. As mask mandates lift and life returns to a new normal, a test to measure immunity would be invaluable to assess individual risk. But so far, a comprehensive test has remained elusive.

According to CNN, one way to assess immunity is to have a blood test that measures the levels of antibodies in your system. And according to the Centers for Disease Control and Prevention (CDC), about 95% of Americans aged 16 and older have antibodies against COVID-19 as of December, the most recent data available. This information was derived from blood donors.

But the Food and Drug Administration warns that testing for antibodies does not correlate to calculating protection against COVID-19.

“There aren’t good correlates of protection — something that says this is the measurement that one needs to know how well they are protected,” said Mehul Suthar, a virologist at the Emory Vaccine Center at Emory University.

While one study found that people who got the Moderna vaccine and had higher antibody levels after vaccination did have a lower risk for COVID-19 infection, experts say not all antibodies are created equal. Only a small amount of these antibodies are considered to be “neutralizing antibodies” capable of preventing infection. Peter Gilbert, professor of vaccine and infectious diseases at Fred Hutchinson Cancer Research Study and the lead author of the Moderna study, said that neutralizing antibodies are a strong correlate for protection against symptomatic infection with COVID-19 and its variants.

The CDC cites one study, which has not been peer reviewed, that shows antibody titers wane more quickly in people who have been vaccinated than in those who have been naturally infected. This is why hybrid immunity, immunity derived from both infection and vaccination, appears to be more durable.

“We know that people who have this hybrid immunity are better protected,” said Marion Pepper, associate professor of immunology at the University of Washington. “So, it really begs the question, ‘which of these parameters is associated with that protection?’”

Some experts say that a more accurate test to measure immunity would involve assessing T cells. Besides antibodies, there are T cells and B cells that contribute to preventing disease, but these are more challenging to measure, say experts. If the virus evades antibodies, the T cells could illicit a strong protective response.

“A memory B cell is a B cell that can get reactivated to make antibodies,” explains Pepper. “It generally doesn’t do that unless a T cell tells it to go, so studying the T cells is going to be really important for understanding this immune protection.” However, she adds that T cells are complex, and it is unlikely we will have a rapid test to measure them any time soon.