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Kidney transplant recipients who didn't mount an antibody response to two COVID-19 vaccine doses might benefit from a third jab, a new study suggested.

In kidney transplant patients with no response following their second dose of the Moderna vaccine, 49% of those who received a third shot saw a serologic response -- defined as antibody levels greater than 50 AU/mL -- after a median 28 days (median antibody titers of responders 586 AU/mL), reported Sophie Caillard, MD, PhD, of Strasbourg University Hospital in France, and colleagues.

However, this still left 51% of kidney transplant patients without a response, the group explained in a research letter.

"The findings in this large group of kidney transplant recipients are in accordance with other studies of solid organ transplant recipients," the researchers pointed out, adding that "the use of a third dose of vaccine may be considered in organ transplant recipients."

However, some kidney transplant patients were more likely to mount a response following a third shot than others.

Specifically, patients who had a weak response after the second dose were more likely to develop an antibody response versus those without an antibody response at all (81.3% vs 27.4%, respectively; mean adjusted difference of antibody titers 894.89 AU/mL, 95% CI 377.41-1,410.37, P=0.001).

Additionally, those taking tacrolimus, mycophenolate, and steroids for immunosuppression were less likely to develop antibodies than those treated with other regimens (35% vs 63%, respectively; mean adjusted difference of antibody titers -697.28 AU/mL, 95% CI -1,193.00 to -201.56, P=0.006).

Of note, there weren't any serious adverse events reported after the third dose.

The study looked at kidney transplant recipients at the outpatient Kidney Transplantation Department of Strasbourg University Hospital from January 20 to June 3. All had a negative history of COVID-19 and SARS-CoV-2 anti-spike IgG levels less than 50 AU/mL. The majority were men, with a median age of 58, and the median time from transplantation was about 5 years.

Most had a deceased kidney donor (77%), and more than half were on tacrolimus plus mycophenolate mofetil/mycophenolic acid and steroids as their immunosuppression maintenance therapy.

One month after receiving their second Moderna dose, 159 kidney transplant patients failed to mount an antibody response. Of this group, 60% had no response at all (titers less than 6.8 AU/mL), and about 40% showed a weak response that still fell under the positivity limit (titers 6.8-49.9 AU/mL).

The standard 100-μg Moderna vaccine dose was administered about 51 days after the second dose.

Response to this booster was measured using Abbott's ARCHITECT IgG II Quant test. According to the manufacturer's label, titers greater than 50 AU/mL were considered positive and correlated with in vitro neutralization of SARS-CoV-2.

"The possibility that patients developed cellular immunity capable of conferring protection against severe disease was not assessed," Caillard's group wrote. "However, the occurrence of severe COVID-19 in some vaccinated transplant recipients may suggest a lack of immunity."

Detailed B-cell and T-cell analyses were not performed, which was a study limitation, they noted.

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