The COVID-19 pandemic is caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and has spread world-wide with millions of cases and hundreds of thousands of deaths to date. The gravity of the situation mandates accelerated efforts to identify safe and effective vaccines. Here, we generated measles virus (MeV)-based vaccine candidates expressing the SARS-CoV-2 spike glycoprotein (S). Insertion of the full-length S protein gene in two different MeV genomic positions resulted in modulated S protein expression. The variant with lower S protein expression levels was genetically stable and induced high levels of effective Th1-biased antibody and T cell responses in mice after two immunizations. In addition to neutralizing IgG antibody responses in a protective range, multifunctional CD8+ and CD4+ T cell responses with S protein-specific killing activity were detected. These results are highly encouraging and support further development of MeV-based COVID-19 vaccines.Author ContributionsCH performed research, analyzed data, and wrote the paper; CS performed research and analyzed data; AA performed research and analyzed data; AE performed research and analyzed data; SM performed research, analyzed data, and wrote the paper; MH developed the bioinformatics pipeline and analyzed data; BS contributed new reagents and concepts; MDM designed and supervised research, analyzed data and wrote the paper; all authors read, corrected and approved the final manuscript.Significance StatementThe COVID-19 pandemic has caused hundreds of thousands of deaths, yet. Therefore, effective vaccine concepts are urgently needed. In search for such a concept, we have analysed a measles virus-based vaccine candidate targeting SARS-CoV-2. Using this well known, safe vaccine backbone, we demonstrate here induction of functional immune responses in both arms of adaptive immunity with the desired immune bias. Therefore, occurrence of immunopathologies such as antibody-dependent enhancement or enhanced respiratory disease is rather unlikely. Moreover, the candidate still induces immunity against the measles, recognized as a looming second menace, when countries are entrapped to stop routine vaccination campaigns in the face of COVID-19. Thus, a bivalent measles-based COVID-19 vaccine could be the solution for two significant public health threats.

SARS-CoV-2 entry is promoted by both cell-surface TMPRSS2 and endolysosomal cathepsins. To investigate the impact of differentially routed virions on host and viral processes, lung epithelial cells expressing distinct combinations of entry factors were infected with authentic viruses. Entry route determined early rates of viral replication and transcription, egress and inhibitor sensitivity, with differences observed between virus strains. Transcriptional profiling revealed that induction of innate immunity was correlated to viral genome and transcript abundance in infected cells. Surface entry triggered early activation of antiviral responses, reducing cumulative virion production, while endolysosomal entry delayed antiviral responses and prolonged virus shedding due to extended cell viability. The likely molecular footprints of escape from antiviral effector targeting were also recorded in viral genomes and correlated with entry route-dependent immune status of cells. TMPRSS2 orthologues from diverse mammals, but not zebra fish, facilitated infection enhancement, which was more pronounced for ancestral strains. Leveraging RNA-seq and scRNA-seq datasets from SARS-CoV-2 infected hamsters, we validate aspects of our model in vivo. In summary, we demonstrate that distinct cellular and viral processes are linked to viral entry route, collectively modulating virus shedding, cell-death rates and viral genome evolution.