1. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a query prefix from a user; obtaining query completions for the query prefix, each query completion having a ranking score, the query completions having a first ranking based on the ranking scores; obtaining a reference query; identifying one or more likely queries that are likely to co-occur with the reference query in user activity sessions; determining that one of the likely queries matches one of the query completions; determining a modified ranking of the query completions including boosting a ranking of the matching query completion; and providing the modified ranking of the query completions in response to receiving the query prefix.