  Advancing the state-of-the-art in large-scale biomedical semantic indexing
and question answering is the main focus of the BioASQ challenge. BioASQ
organizes respective tasks where different teams develop systems that are
evaluated on the same benchmark datasets that represent the real information
needs of experts in the biomedical domain. This paper presents an overview of
the ninth edition of the BioASQ challenge in the context of the Conference and
Labs of the Evaluation Forum (CLEF) 2021. In this year, a new question
answering task, named Synergy, is introduced to support researchers studying
the COVID-19 disease and measure the ability of the participating teams to
discern information while the problem is still developing. In total, 42 teams
with more than 170 systems were registered to participate in the four tasks of
the challenge. The evaluation results, similarly to previous years, show a
performance gain against the baselines which indicates the continuous
improvement of the state-of-the-art in this field.
