Why has the CDC stopped collecting data on breakthrough Covid cases?

It’s difficult to answer these questions because there’s a dearth of rigorous data on breakthrough cases in the United States. The US Centers for Disease Control and Prevention (CDC) tracks only breakthrough cases that lead to hospitalization and death, which it does by gathering data from state health departments. Only 25 states report some data on breakthroughs, and only 15 of those states update it regularly, according to a recent Kaiser Family Foundation analysis of state data. Encouragingly, this data suggests that breakthrough cases among the fully vaccinated are extremely rare – well below 1% in states collecting this information. (Note that undercounts are expected, since people with breakthrough infections may not know they are sick or bother to get tested.)