Detectable plasma severe acute respiratory syndrome coronavirus 2 spike antigen is associated with poor antibody response following third messenger RNA vaccination in kidney transplant recipients.

Background: Kidney transplant recipients (KTRs) generate lower antibody responses to messenger RNA (mRNA)‐based severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) vaccination, yet precise mechanisms for this poor response remain uncertain. One potential contributor is suboptimal spike antigen (sAg) translation and expression owing to transplant immunosuppression, which might lead to insufficient exposure to develop humoral and/or cellular immune responses. Methods: Within a single‐arm clinical trial, 65 KTRs underwent ultrasensitive plasma sAg testing before, and 3 and 14 days after, the third mRNA vaccine doses. Anti‐SARS‐CoV‐2 spike antibodies (anti‐receptor binding domain [anti‐RBD]) were serially measured at 14 and 30 days post‐vaccination. Associations between sAg detection and clinical factors were assessed. Day 30 anti‐RBD titer was compared among those with versus without sAg expression using Wilcoxon rank sum testing. Results: Overall, 16 (25%) KTRs were sAg positive (sAg+) after vaccination, peaking at day 3. Clinical and laboratory factors were broadly similar in sAg(+) versus sAg(‐) KTRs. sAg(+) status was significantly negatively associated with day 30 anti‐RBD response, with median (interquartile range) 10.8 (<0.4–338.3) U/mL if sAg(+) versus 709 (10.5–2309.5) U/mL if sAg(‐) (i.e., 66‐fold lower; p =.01). Conclusion: Inadequate plasma sAg does not likely drive poor antibody responses in KTRs, rather sAg detection implies insufficient immune response to rapidly clear vaccine antigen from blood. Other downstream mechanisms such as sAg trafficking and presentation should be explored. [ABSTRACT FROM AUTHOR]