In the 1990’s, the DTwP vaccine was changed to a DTaP version. The “w” stands for “whole cell” and tended to cause frequent adverse reactions. The change to “acellular”, however, may have afforded a vaccine that “wears out”. The goal of the vaccine is to protect from diphtheria, tetanus, and pertussis. Researchers are reporting that the increase in pertussis may be due to this change. I wonder if this is the only reason, however.