The first part of this instant franchise, 28 Days Later, told of the "Rage virus" outbreak in England, which killed nearly everyone by either becoming a zombie* or being eaten by one. The few who survive do so by their wits and risks, being picked off as the relentless packs cross their paths. A stronghold against the outbreak is found, but human nature being what it is even the safest place is not. Little of 28 Days Laater involves zombies, but that little more than makes up for the disparity via sheer frenetic intensity. Insofar as the ending manages to be happy, it is a happy ending (meh).
In typical sequel fasion, 28 Weeks Later takes the most memorable bits and spends most screen time addressing that material: lots of action, punctuated by intense social drama. This time, however, the standard degrading formula really works as the distilled essence pulls the material into a more powerful and accessable story. Having been revealed in the first story that England, being isolated, did not infect the rest of humanity, we learn some 28 weeks after the initial outbreak that the Rage virus has burned itself out and that London may be rebuilt and repopulated. ...at least until two children return from their well-timed trip abroad (having saved them from the outbreak) and, evading adult authority and containment as children tend to, they return home to find their treasured belongings - and their presumed-dead mother. The virus isn't gone, but there is, for a few, a natural resistance to the disease which must be preserved and exploited at all costs. The remaining tale follows the consequences of this discovery, which is simple and dramatic, and may not be predictable. Those of us who appreciate such horror films will, to be obtuse, look forward to 28 Months Later.
* - A trait of modern zombie movies is to never use the word "zombie".