The next season of Cells at Work! (Hataraku Saibō) won't be a continuation of the story from the main manga series. Instead, the new series set to premiere in January 2021 will cover the events in the spinoff manga series Cells at Work! Code Black (Hataraku Saibō Black), which is one of 5 spinoff series.
The Code Black manga series was started in June 2018, over three years after the main manga series was serialized in Monthly Shōnen Sirius back in March 2015. Code Black is written by Shigemitsu Harada and illustrated by Issei Hatsuyoshi while original Cells at Work manga creator Akane Shimizu serves supervises. In fact, Shimizu doesn't write any of the spinoffs and hasn't written a new chapter of Cells at Work in quite a while as it seems work overseeing the spinoff manga is taking priority.
The next season of Cells at Work! was first announced at last year's AnimeJapan convention and confirmed its Winter 2021 release date back in March. It was originally scheduled to be released in July as part of the Summer 2020 anime season.
Whereas the original Cells at Work series covers the daily life of anthropomorphic cells inside a normal, relatively healthy body, Code Black takes that same premise but changes the setting to that of an out-of-shape, middle-aged Japanese salaryman with a very unhealthy lifestyle. Code Black also focuses on a white and red blood cell but gender swaps the two main protagonists. The spinoff series is also a little bit more risque than the main series and has a 18+ rating in Japan.