*Purpose: Data shows COVID vaccine response after 2 doses in patients on Belatacept immunosuppression (IS) is low, with reported rates of seroconversion (as measured by COVID spike IgG antibody (IgG Ab) detection) of <10%. It is suggested that T cell immunity provides more nuanced marker of immunity. We seek to describe immune response with third dose of vaccine using T cell immunity and spike Ab as surrogate markers.

*Methods: 12 kidney transplant patients on long term belatacept maintenance therapy were included. All patients received induction rabbit anti thymocyte globulin at transplant and were maintained on triple IS with mycophenolate and steroids. All patients received 3 doses of the Pfizer BioNTech SARS CoV2 mRNA vaccine. IgG Ab and T cell immunity response were monitored after 2 doses of vaccine, on the date of 3rd dose with repeat testing done about 4 weeks after 3rd dose. Due to small sample size, T cell response detection was treated qualitatively as “detected” and “negative” results based upon manufacturer instructions (Eurofins Viracor). IgG Ab response was treated qualitatively as “detected” and “negative”, as many responses were too low to be reliably quantifiable.

*Results: Of the 12 included patients, 58% were female, 50% were African American, at mean of 77 months post transplant. After 2 vaccine doses, immunity was detected using the T cell based assay in 6/12 [50.0%, 95% CI: (21.1%-78.9%)]; after 3 doses, T cell immunity detection remained the same (6/12). After 2 doses, IgG was detected in 2/12 patients [16.7%, 95% CI: (2.1%-48.4%)]. After 3 doses, this rate doubled to 4/12 [33.3%, 95% CI: (9.9%-65.1%)]. All IgG Ab detected patients were within the T Cell detected patients. There were statistically significant differences between patients that showed a response vs those that did not although patients with no response had been on a numerically higher duration of belatacept (mean=56 months) vs those with any response (mean=34 months; p=0.23). No patients developed a COVID 19 infection during the study period.

*Conclusions: In this cohort, T cell response identified a bigger subset of patients with vaccine response with 2 mRNA vaccine doses compared with those identified with an IgG response only. However, both T cell immunity and IgG Ab response remained low after 2 or 3 doses, and no patient in in the 2 dose group developed new T cell immunity response after third vaccination. IgG Ab response increased in half of the patients, but these were patients who already had developed a T cell immune response after second dose of vaccine. Total change in COVID spike IgG response after the third dose was up to 33% from an initial 16%, which may demonstrate improved total response to 3 doses. Further research is needed to assess if response rates improve with additional (fourth) doses of COVID vaccine or ‘mix and match’ strategies.

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