The 30 organ transplant recipients in the study had a reduced immune system response after their second dose of the two-shot vaccines compared with the average response of adults in the general population, the data showed.
This is likely due to the fact that, following the procedure, transplant patients are treated with drugs called immunomodulators, which are designed to tamp down the immune system so that it accepts the new organ.
All 30 achieved an adequate antibody response for protection against the virus after being given a booster dose of one of the two vaccines, from either Pfizer-BioNTech or Moderna, with no serious side effects, researchers said.
Because they take immune-modifying drugs, transplant patients need booster doses of vaccines against hepatitis A and B, among others.
Other people on these type of medications, including people with multiple sclerosis and rheumatoid arthritis, may also need extra shots, they said.
"Third doses ... will no doubt be part of the approach to improving vaccine response in immuno-suppressed individuals," study co-author Dr. Dorry Segev told UPI in an email.
"[Our] results are encouraging and give us hope that transplant patients might ultimately be able to achieve a reasonable amount of protection through vaccination," said Segev, director of epidemiology research in organ transplantation at Johns Hopkins University in Baltimore.
Antibodies are cells produced by the human immune system to help fight off a virus. The COVID-19 vaccines, meanwhile, are designed to prime the immune system to create antibodies against the coronavirus, according to the Centers for Disease Control and Prevention.
With these antibodies, the immune system is essentially prepared to fight off the virus, preventing serious infection and illness, the agency says.
Public health officials have suggested recently that booster doses of the COVID-19 vaccine may be needed for full protection, depending on the course of the pandemic, particularly as new variants emerge.
However, it is too soon to tell whether annual shots, as with the flu vaccine, will be needed, experts say.
For this study, Segev and his colleagues studied immune response following vaccination in 30 patients who had received a transplant organ.
After receiving their second dose of the COVID-19 vaccine, 10 participants still had negative antibody levels and all had lower levels than those typically found in the general population, the data showed.
However, after receiving a third or booster dose, their antibody levels rose to levels commonly seen in the general population.
None of the 30 participants given a booster dose experienced serious side effects, with only redness, swelling and soreness at the injection site as well as fever, chills, headache and diarrhea reported.
One of the 30 participants suffered from a "mild" organ rejection, which was resolved with treatment and may or may not have been linked with the vaccine, they said.
While the researchers said the booster dose appears safe for organ transplant patients, they note that more research is needed to establish both the efficacy and safety of a third vaccine dose.
"[Our research] sets the stage for larger studies, where we can better understand which patients will respond well to this approach and who might need a different approach," Segev said.
"In general, we have found the vaccines to be safe in transplant patients, and the safety in our small third dose study also seems acceptable, although larger numbers are needed to be able to say that with more confidence," he said.