PROJECT SUMMARY SARS-CoV-2 has led to unprecedented disruptions to society, killing more than 6 million people worldwide. It is a respiratory virus whose main mode of transmission is via respiratory droplets and aerosols. Social distancing and vaccines have greatly decreased the rate of infection and transmission. Despite these efforts, SARS-CoV- 2 transmission has continued. Moreover, different variants of concern, harboring signature mutations in the virus attachment Spike protein, have emerged. How these changes affect transmission of SARS-CoV-2 in naïve, infected and immunized individuals is not known. The transmission bottleneck is defined as the number of unique virus particles that establish an infection in the recipient host. This number is important as it determines the rate of evolution of the virus and the immune threshold required for protection from infection. This application will use barcoded or tagged SARS-CoV-2 to quantify how many virus particles establish an infection in the recipient host. We will use the Syrian hamster SARS-CoV-2 airborne transmission model to define how the innate and adaptive immune response in the donor and recipient host effect the number of unique transmission events. Using genetically modified hamsters that are deficient in their type I or III interferon response will be used to measure the role of innate immunity on SARS-CoV-2 transmission to the upper respiratory tract (URT) and subsequent dissemination of the virus to lower respiratory tract (LRT). Transmission and dissemination of SARS-CoV-2 will also be quantified in recipient animals that were previously infected with SARS-CoV-2, immunized with mucosal and systemic COVID-19 vaccines, or received neutralizing IgG and IgA antibodies. Finally, this application will measure the impact of immune escape on the transmission bottleneck in immune recipients. Fundamental insights into respiratory virus transmission and dissemination, transmission bottleneck and defining correlates of protection in the URT and LRT will inform future vaccine efforts against respiratory viruses.