Amanda Schaffer provides details on the rise of whooping cough in the US:
Early on, the whooping-cough vaccine was considered an unambiguous success story. Over time, though, scientists—as well as crusading vaccine skeptics—raised concerns about the shots’ side effects, which could include high fever and, occasionally, seizures. In the late nineteen-nineties, the U.S. switched to a new formulation, made not from dead, whole cells of bacteria (as the original had been) but from selected components of the bug that would trigger an immune response more safely. Unfortunately, though, the effectiveness of the updated vaccine waned far faster than the old version had, and faster than researchers had expected. This is probably the main reason that whooping cough has surged recently in older children: those who received the newer vaccine as babies became vulnerable again as the doses that they received between ages four and six, and the boosters that they received between eleven and twelve, wore off. (Parents who refuse to vaccinate their children undoubtedly make the problem worse.)
Higher levels of circulating disease, including among people who had received the shots, meant that the bacteria may have had, in essence, a chance to sniff around.
The Dish 