A third dose of a Covid-19 vaccine may boost protection for some people with weakened immune systems, according to a study published Monday in the Annals of Internal Medicine.

The study, from researchers at Johns Hopkins University, included 30 organ transplant recipients, all of whom had been fully vaccinated with two doses of an mRNA vaccine, either Pfizer-BioNTech or Moderna.

Because organ transplant recipients must take immune-suppressing medications to ensure that their body does not reject the transplant, there’s concern that they may not develop robust responses to the vaccines, leaving them vulnerable to Covid-19.

Indeed, despite being fully vaccinated, the vast majority of patients in the study — 24 patients — had no antibodies against the coronavirus, and six patients had only low levels.

In patients who had no antibodies to begin with, eight had an increase following their third dose of the vaccine.

Segev and his team had found in earlier research that only 17 percent of transplant patients mounted an immune response after one dose of a Covid-19 vaccine.

“I’m pleasantly surprised” that some patients in the new study who hadn’t responded to two doses were able to mount a response after a third dose, Segev said.

Still, as the study demonstrated, an additional dose may not work for every organ transplant patient.

Segev hopes to launch a larger trial to study how a third-dose booster regimen works in transplant patients across the country and is hoping to enroll 1,000 patients.

“The mantra I'm telling my transplant patients is get vaccinated and act as if you are unvaccinated,” he said