An Ohio man registered "no antibodies whatsoever" after four COVID-19 shots, per CNN.
Andrew Linder, who is immunocompromised, is isolating and limiting contact with people.
The antibody test he took has some limitations as a proxy for immunity.
An immunocompromised man had an antibody test came back negative even after receiving four doses of the COVID-19 vaccine, CNN reported Tuesday.
"I had no antibodies whatsoever. That was shocking and scary and sucky for sure," Andrew Linder, a kidney transplant recipient in Akron, Ohio, told the network.
Because he is not sure that he is protected against the coronavirus, Linder, 34, is still staying at home almost all the time and strictly limiting his contact with people other than his wife, per CNN.
Linder is taking immunosuppressants, drugs commonly used after transplants to stop the body rejecting a new organ.
"I almost feel just as unsafe or if not potentially a little bit more unsafe now than at the beginning of the pandemic, just for the fact that I could get it at this point in time," Linder told CNN.
It was not immediately clear how Linder assessed his antibody level. A common way is via an antibody test, though these have been criticized as an imperfect tool.
The Food and Drug Administration advises against using them to check immunity.
Antony Fauci, the White House chief medical advisor, previously told Insider's Hilary Brueck, that there are problems with them as a measure of whether the body responded properly to a vaccine.
One reason is that the body's protection against the virus is complex, and having no measurable antibodies in the blood doesn't necessarily mean that the shots are not providing protection.
Some immunocompromised people, who are at increased risk of severe disease from COVID-19, are turning to the tests to give them some sense of reassurance that the shots have worked, Scientific American reported.
Vaccines remain the best protection against hospitalization and death from COVID-19, even among immunocompromised people.
But early data suggested that two shots of vaccine might provide less protection for immunocompromised people.
A CDC study published last week found that two doses of Pfizer and Moderna vaccines reduced the risk of hospitalization by 77% among immunocompromised adults, compared to 90% among immunocompetent adults.
Last month, the Centers for Disease Control and Prevention (CDC) recommended most adults with compromised immune systems can receive up to four doses of Pfizer of Moderna COVID-19 vaccines - the guidance is slightly different for those who got a first shot of J&J vaccine.