In light of recent news that antibody levels may wane six or so months after vaccination, people have started taking antibody tests to gauge their immunity against COVID-19.
It’s tempting: Booster shots are available for many people, and the hope is that an antibody test — which involves a quick blood draw — could provide some clues as to whether or not you may be due for another vaccine.
The general consensus among infectious diseases specialists is that it’s far too early for people to be making serious decisions off of antibody tests results. The Food and Drug Administration stated in May that these tests should not be used to measure a person’s immunity after vaccination. The data is limited, and antibody tests don’t show the full picture of how durable our immune response is.
That said, scientists have uncovered some important clues about antibody levels and the protection they provide against symptomatic infection. But, still, we don’t have enough data to declare a person is protected if they have a certain amount of antibodies in their system.
“The antibody tests are fun, but they are limited,” Adam Ratner, the director of pediatric infectious diseases at NYU Langone Health, told HuffPost.
For example, there is no universal standard for antibody tests — results may fluctuate depending on what company’s test you take. But they are usually within the same general ballpark, said Monica Gandhi, an infectious diseases specialist with the University of California, San Francisco.
When are antibody tests useful?
There are various types of antibody tests out there — qualitative, which will simply tell you whether or not you have antibodies, and quantitative, which put a number to how many antibodies you have.
Qualitative tests — the ones that result in either a plus or a minus — are helpful in determining if you’ve been infected in the past, but they aren’t useful when it comes to evaluating vaccine-induced immunity.
“Even if you check an antibody quantitatively and tell me your number, I don’t know what to tell you — is it enough, is it going to protect you, is it too low? — we just don’t have that data yet.”
There are also antibody tests that measure antibodies against the spike protein (which is what the vaccines are designed to teach our immune system to attack) and others that measure antibodies against an element called the nucleocapsid (which are helpful in identifying if someone previously had COVID-19, but they fall short at detecting antibodies in someone who’s only been vaccinated).
For the purpose of vaccinated people wanting to evaluate their antibodies, we want to talk about quantitative antibody tests that measure antibodies to the spike protein.
The main dilemma with quantitative antibody tests is that we don’t know exactly how many of those antibodies you need for protection against COVID-19.
“Even if you check an antibody quantitatively and tell me your number, I don’t know what to tell you — is it enough, is it going to protect you, is it too low? — we just don’t have that data yet,” said Onyema Ogbuagu, a Yale Medicine infectious diseases specialist.
With other infections, higher antibodies correlate to stronger protection, and it won’t be unusual if that’s also the case with COVID-19, Ogbuagu said. But we really need more data to understand the link with the coronavirus specifically.
There is one pre-print study that offers some clues. The research specifically looked for correlates of protection against symptomatic COVID-19 and found that vaccine efficacy against symptomatic infection was 90.7% when antibody levels were at 100 u/mL in lab results and increased to 96.1% when antibodies were at 1,000 u/mL.
But that’s just one study, and we need many more. “There is still not a really robust correlate of immunity, meaning a level of antibody where if you’re above that level we can confidently say, ‘yes, this person is protected,’” Ratner said.
At this point, experts largely agree that antibody tests may be most useful for determining prior infection and identifying if an immunocompromised person mounted an immune response to the first round of shots or after the booster, Gandhi said.
What antibody tests can’t tell us
Antibody tests fail to measure the second, very important arm of the immune system: the T-cells (which clear infected cells and help prevent an infection from becoming severe) and B-cells (which produce new antibodies targeting the variant upon exposure). Because of that, “we need to be very cautious of how we interpret antibodies waning over time,” Ogbuagu said.
Antibody levels will inevitably decline over time, but your body will keep a memory of the virus. Even if an antibody test shows that you have low antibodies, you could still be very well protected in the long term against severe illness, hospitalization and death thanks to your T-cells and B-cells.
Antibody tests also don’t tell us the quality of the antibodies that they detect. And some evidence shows that as antibody levels decline, they are gradually replaced by higher-quality antibodies, suggesting that eventually, it takes fewer antibodies to stay protected. “Waning antibodies don’t tell the full story,” Gandhi said.
Again, there’s a lot we still don’t know about antibodies and protection. According to Ratner, the vast majority of vaccinated people will have some amount of detectable antibodies in their system, but we are still figuring out how they correlate to protection and when they indicate it’s time for a booster.
“What we do know is that, in aggregate, those people are mostly protected against severe disease,” Ratner said.
Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.