Theel said she sympathizes with the temptation to do the occasional antibody check, whether after a suspected infection or a vaccine. When she posted an informal poll to Twitter last month, more than 80 percent of the 118 people who responded said they’d be interested in testing themselves for antibodies after their jabs. “I think it’s a psychological thing,” Theel said. “You injected something in my body. I want to know: Did I respond?”
Antibodies do make up a crucial component of the immune system, especially when it comes to warding off viral disease. But positive tests for coronavirus antibodies have no bearing on whether these molecules will actually quash the virus in a more natural setting, or on how long their protective powers will last. And researchers don’t yet know what antibody levels are needed to ward off infection or disease. “It’s hard to use antibodies as a thing saying, ‘I’m protected from future disease,’” Nahid Bhadelia, an infectious-disease physician and the medical director of the special-pathogens unit at Boston Medical Center, told me. The portrait of protection has grown only more complex as the coronavirus has continued to splinter into variants, some of which appear to evade the pathogen-purging powers of certain antibodies.
For people who are still keen on testing themselves postvaccination, Theel and Babady both recommend waiting until about a week or two after the second shot and seeking out a test that detects a type of antibody called immunoglobulin G, or IgG, that will recognize the coronavirus’s spike protein. And a negative result shouldn’t necessarily cause alarm. Some tests are better than others at picking up low levels of antibodies. Not everyone will make enough antibodies to trip a typical test, but they could still be well protected against the virus, thanks to the many other immune molecules and cells our bodies produce.
Antibody tests do have a role to play in research. Public-health experts routinely use them to track where the coronavirus has silently spread. Measured in a lab, neutralizing antibodies can also aid the development of vaccines or treatments such as convalescent plasma and monoclonal antibodies. And as new variants of the coronavirus continue to spring up around the globe, antibody studies will help scientists suss out how well antibodies generated in response to one version of the virus will work against another.
Last week, two preprint studies seemed to hint at yet another use for antibody tests: a prevaccination screening strategy for people who have already been infected by the coronavirus. With antibodies that recognize the virus already teeming in their blood, these people might not need their second dose of the Pfizer-BioNTech or Moderna vaccines, some researchers argue. “Scientifically, it might make sense,” Theel said. “You’re basically boosting the immune system with the first dose, after a previous infection.” But with so much ambiguity about how many antibodies are needed for protection, Theel is still cautious about endorsing the idea. Doling out two shots to everyone, regardless of antibody status, remains the safest bet, since the second injection doesn’t seem to pose any threat to COVID-19 survivors.
A year into the pandemic, the consensus on antibody tests is largely the same as it was months ago: A positive result “shouldn’t change anyone’s behavior,” Bhadelia said. The unknowns about immunity have shrunk. But they’re still too big to gamble with.