Stable SARS-CoV-2 antibody levels and functionality in serum and COVID-19 convalescent plasma after long-term storage

The coronavirus disease 2019 (COVID-19) pandemic necessitated various therapeutic approaches, including convalescent plasma (CP) administration. The administration timing of COVID-19 CP (CCP), antibody specificity and quantity were identified as crucial factors for therapeutic success. Currently, antibody durability and storage time are still under debate. The aim of this study was to evaluate the stability and in vitro functionality of severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) antibodies in human plasma and serum after long-term storage, to provide a framework for generally applicable rules regarding the long-term storage of CCP.

Serum and plasma samples of CCP donations were investigated at the time of donation and after 2 and 3 years' storage at less than -30°C using (electro)chemiluminescence immunoassays and enzyme-linked immunosorbent assays, with the plasma undergoing multiple freezing and thawing.

Our data reveal robust levels of SARS-CoV-2 antibodies after long-term storage. Furthermore, our findings also indicate that multiple freezing and thawing cycles do not affect the antibody levels or their neutralizing capability.

As antibody stability and in vitro functionality are maintained over extended periods, even after repeated freezing and thawing, our findings support long-term storage of CCP, particularly benefiting vulnerable populations such as immunocompromised individuals. By now, donors have likely encountered various SARS-CoV-2 variants and vaccine-acquired antibodies. This antibody mix present in CCP is suggested to protect even against new variants. Our data indicate that current regulations for the storage of CCP can be extended and that CCPs could be used for therapeutic purposes after long-term storage without significant loss of antibody quantity.