MUM (Multitask Unified Model)
Multimodal, multilingual model announced at Google I/O 2021 and first applied to complex informational queries.
What changed
Google announced MUM as a model trained across 75 languages and multiple content types, described as far more capable than BERT at answering complex, multi-part questions. Its first production use was surfacing information about COVID-19 vaccine names across languages, and it later powered features such as "things to know" panels and refinement chips. Google said MUM was not used as a general ranking signal in the way BERT was, and it is listed among Google's ranking systems.
Timing
Rollout began 18 May 2021. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.