--- title: "MUM (2021): what Google's Multitask Unified Model does in Search" url: https://seomadman.com/updates/mum-2021 section: updates language: en published: 2026-09-26T00:00:00.000Z modified: 2026-09-26T00:00:00.000Z author: Adam Hafez topics: ["AI search"] --- # MUM (2021): what Google's Multitask Unified Model does in Search ## The short answer MUM, the Multitask Unified Model, is an AI technology Google announced on 18 May 2021, not a ranking update with a rollout date. Its first use in Search, in June 2021, found over 800 names for COVID-19 vaccines in more than 50 languages. Google says MUM is not currently used for general ranking in Search. ## Key takeaways - Google announced MUM on 18 May 2021 as a new technology, with features promised over months and years. - Google described MUM as 1,000 times more powerful than BERT and trained across 75 languages. - Its first Search use identified over 800 vaccine name variations in more than 50 languages. - Google's ranking systems guide says MUM is not currently used for general ranking in Search. - There is no MUM-specific optimization; Google's helpful content guidance applies as usual. ## What Google changed MUM was not a ranking update. Google announced the Multitask Unified Model on 18 May 2021 as a technology it would bring to Search over time: "We'll bring MUM-powered features and improvements to our products in the coming months and years." There was no rollout window and no ranking change tied to the announcement. In that post, Google's Pandu Nayak said MUM uses the T5 text-to-text framework and "is 1,000 times more powerful than BERT." Google wrote that MUM both understands and generates language, is trained across 75 languages and many tasks at once, and is multimodal, handling text and images. ## When it rolled out The announcement date, 18 May 2021, is the date on this page because it is the date Google first described MUM. Google described MUM's first application in Search on 29 June 2021. That was a specific task, not a general ranking change: identifying the names people use for COVID-19 vaccines, so Search could show trusted information for all of them. ## What is known about its impact Google gave one figure, for that first task: "With MUM, we were able to identify over 800 variations of vaccine names in more than 50 languages in a matter of seconds." Google said that work would otherwise take hundreds of human hours. It published no figures on ranking or traffic, because MUM was not applied to general ranking. ## What site owners should do Google's current ranking systems guide lists MUM as an AI system "capable of both understanding and generating language," and says it is "not currently used for general ranking in Search." It names specific uses instead, such as vaccine searches and featured snippet callouts. MUM is still listed as an active system, not a retired one. - There is nothing to optimize for MUM directly. - Write for people first, as Google's helpful content guidance describes. - Cover a topic in the language your audience searches in, clearly and completely. ## How to check whether it affected you MUM has no ranking rollout to measure against, and Search Console keeps only 16 months of data in any case. If you are reviewing traffic from mid-2021, attribute changes to the dated updates around it instead, such as the June 2021 and July 2021 core updates. The [Google Algorithm Updates timeline](/updates) lists them with their dates. ## Related updates - [BERT (2019)](/updates/bert-2019) was the language model Google compared MUM against. - [Passage ranking (2021)](/updates/passage-ranking-2021) was another AI system Google applied to Search that year. - The [June 2021 core update](/updates/june-2021-core-update) rolled out in the same period. ## Frequently asked questions ### Was MUM a Google ranking update? No. Google announced MUM as a technology, an AI model it would apply to Search features over time. Its ranking systems guide says MUM is not currently used for general ranking in Search, only for specific applications. ### When did Google launch MUM? Google first announced MUM on 18 May 2021. It described its first application in Search, identifying COVID-19 vaccine names, on 29 June 2021. There was no single rollout date for rankings. ### What is MUM used for in Google Search? Google's ranking systems guide names specific applications, such as improving searches for COVID-19 vaccine information and improving the featured snippet callouts Google displays. ### How is MUM different from BERT? Google said MUM is 1,000 times more powerful than BERT, uses the T5 text-to-text framework, both understands and generates language, is trained across 75 languages and is multimodal, handling text and images. ## Sources 1. [MUM: A new AI milestone for understanding information](https://blog.google/products-and-platforms/products/search/introducing-mum/) - Google Keyword Blog (primary) 2. [How MUM improved Google Searches for vaccine information](https://blog.google/products-and-platforms/products/search/how-mum-improved-google-searches-vaccine-information/) - Google Keyword Blog (primary) 3. [A guide to Google Search ranking systems](https://developers.google.com/search/docs/appearance/ranking-systems-guide) - Google Search Central (primary) 4. [Creating helpful, reliable, people-first content](https://developers.google.com/search/docs/fundamentals/creating-helpful-content) - Google Search Central (primary)