  We present Covidex, a search engine that exploits the latest neural ranking
models to provide information access to the COVID-19 Open Research Dataset
curated by the Allen Institute for AI. Our system has been online and serving
users since late March 2020. The Covidex is the user application component of
our three-pronged strategy to develop technologies for helping domain experts
tackle the ongoing global pandemic. In addition, we provide robust and
easy-to-use keyword search infrastructure that exploits mature fusion-based
methods as well as standalone neural ranking models that can be incorporated
into other applications. These techniques have been evaluated in the ongoing
TREC-COVID challenge: Our infrastructure and baselines have been adopted by
many participants, including some of the highest-scoring runs in rounds 1, 2,
and 3. In round 3, we report the highest-scoring run that takes advantage of
previous training data and the second-highest fully automatic run.
