Long-term antibody titers variation in unvaccinated patients receiving convalescent plasma or placebo for severe SARS-CoV-2 pulmonary infection.

Background: Convalescent plasma (CP) became a prominent treatment in the early stages of the SARS-CoV-2 pandemic. In Argentina, a randomized clinical trial was executed to compare the use of CP in inpatients with severe COVID-19 pneumonia versus placebo. No differences in clinical outcomes or overall mortality between groups were observed. We conducted a cohort study in outpatients enrolled in the trial to describe long-term antibody titer variations between CP and placebo recipients. Methods: Patients' total SARS-CoV-2 IgG antibodies against spike protein were collected 3, 6 and 12 months after hospital discharge from August 2020 to December 2021. In addition, reinfections, deaths and vaccination status were retrieved. Statistical analysis was performed using antibody geometric mean titers (GMT). All estimations were made considering the date of the trial infusion (placebo or CP) as time 0. Results: From the 93 patients included in the follow-up, 64 had received CP and 29 placebo. We excluded all 12 month measurements because they were collected after the patients' vaccination date. At 90 days post-infusion, patients had an antibody GMT of 8.1 (IQR 7.4-8.1) in the CP group and 8.8 (IQR 8.1-9.1) in the placebo group. At 180 days, both groups had a GMT of 8.1 (IQR 7.4-8.1). No statistical differences in GMT were found between CP and placebo groups at 90 days (p = 0.12) and 180 days (p = 0.25). No patients registered a new COVID-19 infection; one died in the CP group from an ischemic stroke. Conclusions: No differences were observed in long-term antibody titers in unvaccinated patients that received CP or placebo after severe COVID-19 pneumonia.