The German company decided to completely end the trip on Sunday, December 19. While ~1300 passengers had completed the contracted 7-day voyage normally, another ~300 had to prematurely abandon their B2B (7+7-day) journey.
The outbreak was originally detected among 4 passengers. After doing a massive test, "some more cases" were added, as explained by TUI. All of them, and their close contacts, were isolated in the designated area on the ship. Once they arrive in Gran Canaria, they are keeping the corresponding quarantine.
For more Mein Schiff 4 incidents and accidents see the ship's CruiseMinus page.