A vision for estimation of the instantaneous reproductive number.

The reproductive number, Rt, is a popular metric used for monitoring infectious diseases. Rt describes the expected number of infections that will be generated from a single infection at time t, which maps nicely to the likelihood that disease incidence will increase, decrease, or remain constant in the near future. Although this metric has existed for decades, it became more widely used during the COVID-19 pandemic and there was a subsequent proliferation of new estimation methods and software tools. This rapid development of methods and tools presents many opportunities and challenges for users, researchers, and decision makers. In recognition of this growth, we convened a three-day "collabathon" in September 2024 to bring together researchers and public health practitioners to identify challenges and areas for future development in Rt estimation and to begin work in these areas. Here we provide a high-level summary of current methods and report on the findings from the collabathon, including a summary of current challenges and recommendations for future development, evaluation and interpretation of Rt.