A Third Dose of SARS-CoV-2 Vaccine Increases Neutralizing Antibodies Against Variants of Concern in Solid Organ Transplant Recipients

Abstract Immunocompromised populations are at high risk for severe COVID-19. Vaccine-induced SARS-CoV-2 antibody responses are attenuated in solid organ transplant recipients (SOTRs), and breakthrough infections are more common. Additional SARS-CoV-2 vaccine doses may increase anti-spike antibody titers in some SOTRs, but whether this results in enhanced neutralizing capability, especially versus novel variants of concern (VOCs) that exhibit immune escape and higher infectivity (e.g., the Delta variant), is unclear. Here, we report that a third dose of a SARS-CoV-2 vaccine increases anti-SARS-CoV-2 spike and RBD IgG levels as well as plasma neutralizing capability versus VOCs, including Delta, in some SOTRs. However, anti-spike IgG and neutralizing capability remained significantly reduced compared to fully vaccinated healthy controls. These findings highlight the need for continued study of strategies to improve protection from COVID-19 in immunosuppressed populations as more SARS-CoV-2 VOCs emerge.