Searching for scientific evidence in a pandemic: An overview of TREC-COVID

•TREC-COVID Information Retrieval (IR) shared task, developed in response to the COVID-19 pandemic.•Differs from most IR shared tasks (multi-round, short turnaround, residual judgments, feedback).•Significant participation in the task, including 92 unique teams and 556 total submissions.•Final test collection contains 50 topics with 69,318 manual judgments on the CORD-19 corpus.