Background: After basic immunization with 2 mRNA SARS-CoV-2 vaccine doses, only a small proportion of patients who are severely immunocompromised generate a sufficient antibody response. Hence, we assessed the additional benefit of a third SARS-CoV-2 vaccine in patients with different levels of immunosuppression.

Methods: In this observational extension of the COVERALL trial (Corona Vaccine Trial Platform), we recruited patients from the Swiss HIV Cohort Study and the Swiss Transplant Cohort Study (ie, lung and kidney transplant recipients). We collected blood samples before and 8 weeks after the third SARS-CoV-2 vaccination with either mRNA-1273 (Moderna) or BNT162b2 (Pfizer-BioNTech). The primary outcome was the proportion of participants showing an antibody response (Elecsys Anti-SARS-CoV-2 S test; threshold ≥100 U/mL) 8 weeks after the third SARS-CoV-2 vaccination. We also compared the proportion of patients who reached the primary outcome from basic immunization (the first and second vaccines) to the third vaccination.

Results: Nearly all participants (97.2% [95% CI, 95.9%-98.6%], 564/580) had an antibody response. This response was comparable between mRNA-1273 (96.1% [95% CI, 93.7%-98.6%], 245/255) and BNT162b2 (98.2% [95% CI, 96.7%-99.6%], 319/325). Stratification by cohort showed that 99.8% (502/503) of people living with HIV and 80.5% (62/77) of recipients of solid organ transplants achieved the primary endpoint. The proportion of patients with an antibody response in solid organ transplant recipients improved from the second vaccination (22.7%, 15/66) to the third (80.5%, 62/77).

Conclusions: People living with HIV had a high antibody response. The third vaccine increased the proportion of solid organ transplant recipients with an antibody response.Clinical Trials Registration. NCT04805125 (ClinicalTrials.gov).