After 28 Days Later and 28 Weeks Later, the natural progression would seem to be 28 Months Later. But screenwriter Alex Garland says that you shouldn’t expect such a movie to happen any time soon, or even at all.
During a Reddit AMA session he said that there previously have been ‘kind of’ plans about where the franchise could go, but right now he says: “[I] doubt it will ever happen”.
Which would be a pity because the first two films were pretty decent, even if you don’t like their introduction of crazed running zombies, which a ton of movies used afterwards, including the Dawn of the Dead remake (which was also good).
Anyway, 28 Months Later = $$$, so I’d guess that it’s going to happen with or without Garland’s involvement…