Impact of Emerging SARS-CoV-2 Variants on Humoral Immune Responses of COVID-19-Infected and Vaccinated Individuals

Abstract

The COVID-19 pandemic caused by the previously unknown SARS-CoV-2 presented the scientific community with the challenging task of understanding the basic properties of the virus under rapidly evolving and changing circumstances, in order to pave the way for effective therapies and vaccines. To assess the humoral immune response to SARS-CoV-2, we developed and validated a multiplex serological immunoassay, MULTICOV-AB, for the reliable detection of antibodies against SARS-CoV-2 key antigens including the receptor binding domain (RBD) and nucleocapsid, demonstrating high sensitivity and specificity for the classification of infection. As the seroprevalence of SARS-CoV-2 in the population continuously increased and vaccination campaigns progressed, the quality of the antibody response, including longevity and immune protection, became the focus of research. This has been reinforced by the continuous emergence of new SARS-CoV-2 variants that differ in their ability to evade existing immune responses and thus make reinfection more likely. Therefore, we have developed RBDCoV-ACE2, a multiplex surrogate neutralisation assay that investigates the presence of neutralising antibodies (NAbs) that interfere with the binding of the SARS-CoV-2 RBD to the host cellular receptor angiotensin-converting enzyme 2 (ACE2), which is equivalent to preventing infection in vivo. Using both assays, we were able to show that variants with the E484K mutation, for example Beta, Gamma and Theta, are more likely to evade existing humoral immune responses, as antibodies formed against the RBD of the wild-type virus exhibited up to 14-fold lower ACE2 binding inhibition against these variants. Furthermore, we were able to compare vaccination regimens and show that mRNA-based vaccination led to a superior humoral immune response compared to homologous vector-based vaccine regimens. We were able to show that the long-term humoral immune response to SARS-CoV-2 infection lasts longer in children than in adults, even after asymptomatic infection. With the emergence of the highly mutated Omicron variants, we were able to show that both antigen binding and ACE2 binding inhibition of antibodies induced by vaccination or/and infection were greatly reduced, with booster doses and breakthrough infections causing a significant increase in both levels. This reduction in neutralising capacity was particularly noticeable in serum from immunocompromised individuals, such as hemodialysis or inflammatory bowel disease (IBD) patients, where we were able to contribute indications for future vaccination strategies. Both MULTICOV-AB and RBDCoV-ACE2 continue to provide valuable knowledge about the humoral immune response to novel emerging SARS-CoV-2 variants.