Among patients in the study who had no measurable antibodies after receiving two doses of vaccine, one-third of them saw a rise in antibodies after a third dose -- and among those with low antibody levels after two doses, all of them saw an increase after a third dose.

"We don't know if you need the same off-the-charts level of antibodies that people with normal immune systems have."

When pharmaceutical companies tested coronavirus vaccines in clinical trials last year, they specifically excluded people who were taking immunosuppressive drugs due to potential risks.

But for transplant patients, after a two-dose full vaccine series, "the overwhelming majority have either no antibodies or low antibodies," Segev said.

Boosting antibody levels

The researchers, from the Johns Hopkins University School of Medicine in Baltimore, examined antibody responses and vaccine reactions in 30 organ transplant patients who received a third dose of coronavirus vaccine between March and May of this year.

Fifteen patients received a dose of the Johnson & Johnson vaccine, nine received Moderna's shot and six received Pfizer's around 67 days after completing the second dose of their original vaccination.

The researchers tested the patients for antibodies before they received their third doses of vaccine.

"And what's happening is transplant patients are getting the vaccine series, they're getting their antibodies checked, they're realizing that they're low, they're talking to their doctors, and then they're going and getting a third dose to try to boost this."

Various medical societies, such as the American College of Rheumatology, the American Society of Transplantation, and the International Organization for the Study of Inflammatory Bowel Disease, essentially say the antibody tests won't fully answer patients' questions about whether their vaccinations worked.

The US Food and Drug Administration and the US Centers for Disease Control and Prevention also advise against checking antibodies.

The National Institutes of Health is setting out to determine what approaches might work best if the vaccine is failing people who are immune compromised.