The United States is on track to have the most severe whooping cough outbreak in a half-century, with 18,000 cases already diagnosed this year resulting in nine deaths. All of this is happening despite the fact that we've had a whooping cough, or pertussis, vaccine for about 70 years.

The death rate is still significantly lower than in the 1920s, when about 6,000 children would die annually from pertussis. How does an outbreak happen when we know how to immunize against the disease?  Raymond Cattaneo, a pediatrician based in Kansas City, Mo., has a few thoughts on the subject. Here are his two of his explanations for what's happening:

2. Too many people are refusing immunizations. The herd is real and it is vitally important to the success of immunizations. Because no immunization is 100% effective, plus the fact that some people CAN’T receive immunizations, 85%-95% of the population needs to be immunized to protect all of us. When too many people fail to immunize their children against preventable diseases, those viruses and bacteria are able to break through growing holes in the herd and cause worse and worse outbreaks. That is definitely what we are seeing now with pertussis. But, it has also happened with measles and mumps.


4. The new version of the pertussis immunization is not as effective as the old version. Without getting too technical, the early versions of the pertussis immunization utilized whole bacterial cells when formulating the suspensions (DTP). While the immunization was highly effective, it unfortunately caused expected, but severe side effects. In the late 1990′s, versions of the immunization using limited antigen numbers and no part of the cell (i.e. acellular) were licensed (DTaP). While these formulations are effective (and definitely cause less side effects), they are not as effective as the previous version of the immunization.

Others have cited the recent wave of public health budget cuts, which have eliminated 20 percent of the workforce since 2008 and weakened health departments' ability to respond to outbreaks, as one other explanation.