Vaccine-Resistant Germ Might Cause Whooping Cough

A recent study says that some cases of whooping cough in the U.S. could be caused by a vaccine-resistant germ.

The bacteria that causes whooping cough has mutated over time, seen here through an electron microscope.

Researchers examined the bacteria that caused 12 cases of whooping cough in Philadelphia.

Researchers found the bacteria had mutated since it was used to create the highly-effective vaccine developed in the 1990's and widely used today. 

This year has been the worst for whooping cough in about 60 years.

Cases of whooping cough have been on the rise recently in the U.S. and other countries, according to the study published in the New England Journal of Medicine.

Researchers wrote that more studies must be conducted to determine if this is a widespread phenomenon and if it is the cause of the increase in infections.

Whooping cough, or Pertussis, causes a persistent and violent cough. It can be treated with antibiotics, and infections usually last six weeks.

The Associated Press contributed to this report.