Whooping cough - or pertussis, as the disease is officially called - has been making a comeback in recent years. In 2010, there were 27,550 cases. This year was supposed to have fewer cases, but instead it looks to be on track to be even worse, with 26,146 cases to date. The question of why the disease has returned as a public health concern has haunted doctors. Now, a study published in the New England Journal of Medicine reports that the primary reason for these outbreaks may lie in the vaccine itself.

The DTaP vaccine - diphtheria, tetanus, and acellular pertussis - was first introduced to the American public in 1991. The vaccine currently uses purified portions of the pertussis bacterium to provoke immunity.

The previous vaccine, introduced in the 1940s, used a dead microbe to trigger immunity. Though it was extremely effective, it also came with the risk of severe side effects, allergic reactions and neurological damage.

Nicola Klein, co-director of the Kaiser Permanente Vaccine Study Center, and her colleagues found that the efficacy of the DTaP vaccine decreases after the fifth dose. In a study conducted among 277 children between the ages of 4 and 12, who had all received the vaccine, researchers found that, of the children who tested positive for the disease, they had received the vaccine on average 1,699 days prior to the study.

For comparison, kids who had tested negative had received their last dose of the vaccine an average of 1,028 days prior. What's more, researchers found that the vaccine's efficacy typically reduced 42 percent every year. If a vaccine was 95 percent effective immediately after the doses, after five years, it would be 72 percent effective.

While doctors say that there was no idea during trials that the whooping cough vaccine's efficacy would wane like it has, some doctors say that there were indications that the DTaP vaccine was less effective than its predecessor. The DTaP vaccine is 85 percent effective. No vaccine is 100 percent effective and it is not altogether surprising that immunity starts to fade. Doctors also say that, even though children between the ages of 5 and 13 have been disproportionately affected by these outbreaks, poor diagnosis of the illness and the lack of adult immunization may also be to blame.

Nevertheless, 85 percent efficacy is better than 0 percent. Though it is unclear exactly how much protection the vaccine provides, it is clear that the DTaP vaccine is simply a stopgap before the vaccine will need to be switched.

Whooping cough typically begins with common cold-like symptoms, with perhaps a cough and a fever. Symptoms usually progress after one to two weeks to severe coughs that can last for weeks. Adults with whooping cough may suffer from cracked ribs; infants, who are at particular risk because they are unvaccinated, can die from the disease.