The only thing that has significantly slowed Plague Inc. this year is the Chinese government. In late February, as the number of reported Covid-19 cases in China neared 80,000, regulators banned the game for “illegal” content, according to a notice shared online by Ndemic Creations. Plague Inc. has not been sold there since, and Mr. Vaughan said his studio had not received more explanation for the ban. “You could draw your own assumptions on it,” he said.

In Plague Inc.’s absence, numerous copycats have cropped up, according to Daniel Ahmad, an analyst at Niko Partners and a longtime observer of the Chinese game market. China has also seen a burst of explicitly anti-Covid-19 games, he said. One, called Battle of Pathogens, replaces the fruit of Fruit Ninja, a popular fruit-slashing game, with pathogens to slice.

Mr. Vaughan laments Plague Inc.’s fate in China, but he has spent his energy working on a new mode of the game that will flip the dynamic and let players fight the spread of a pandemic. It is something he said he started thinking about before he released the game and is motivated to make it now. Work on this new mode is delaying other ideas for Plague Inc., including a plan to work vaccine opponents into the game, which Mr. Vaughan wanted to implement in response to a popular online petition last year. (His studio paired an announcement in March about the new mode with a pledge to donate a quarter-million dollars to organizations fighting Covid-19.)

The game developer intends to consult with disease experts and he expects this new way of playing Plague Inc. to let people role-play their own pandemic response. “You’ve got to tread between reacting not enough and reacting too much to an outbreak,” he said. “And you’re never going to know what the right answer was until afterwards.”

While he is critical of real-world governments for reacting too slowly to the spread of Covid-19, he speculated that in this mode, players may find that locking down society too soon might impair the ability of people to feed themselves or for society to find a cure.

Mr. Vaughan is not sure when he will finish making the new mode. He is not even sure how all of its underlying systems will function: “It’s a bit like taking an old clock apart using your mind and having all the bits floating around in the air in front of you, and then putting it back together a different way and seeing if it works.”