Despite the success of current COVID-19 vaccines, the immunity they generate wanes over time, requiring periodic boosters. The limited durability of memory responses, particularly from mRNA vaccines, remains a major challenge for achieving long-term protection. Developing vaccines that induce more sustained immunity would lessen the need for frequent revaccination, improving global vaccination logistics, especially in resource-limited settings. DNA-launched self-amplifying RNA replicons (DREP) and modified vaccinia virus Ankara (MVA) vectors are promising vaccine platforms capable of inducing potent humoral and cellular immunity. In this study, we evaluated SARS-CoV-2-specific immune responses in C57BL/6 mice following homologous and heterologous prime/boost regimens combining DREP- and MVA-based vaccines expressing the spike (S) protein from either the ancestral Wuhan strain or the Omicron XBB.1.5 variant. Homologous (DREP/DREP, MVA/MVA) and heterologous (DREP/MVA) regimens were followed for six months. MVA-S(3P)-based boosters elicited robust and durable anti-S IgG antibodies cross-recognizing multiple variants, with minimal decay over time. Neutralization mapped to the booster antigen: MVA-S(3PWuhan) induced neutralization of the ancestral strain, while MVA-S(3PXBB.1.5) selectively neutralized Omicron subvariants, maintaining high titers for at least six months. MVA-S(3P) boosters also enhanced antibody Fc-effector functions, memory B cells, and T follicular helper responses. Early after boosting, heterologous DREP/MVA regimens induced stronger CD4⁺ and CD8⁺ T-cell responses, while at six months all MVA-S(3P)-boosted groups maintained superior, long-lived cellular immunity. Collectively, MVA-S(3P)-based boosters improved the magnitude, breadth, and durability of humoral and cellular responses, supporting their strategic use in homologous and heterologous DREP/MVA vaccination regimens against SARS-CoV-2 and emerging variants.
Pérez et al. (Thu,) studied this question.