By Selena Hill ( | First Posted: Feb 08, 2013 09:22 PM EST

French professor Erwann Loret works on samples of a vaccine against AIDS in his laboratory in Marseille January 29, 2013. Clinical trials of a vaccine against the AIDS virus (HIV-1) will begin in February in Marseille with forty-eight infected volunteers, the Public Assistance Hospitals of Marseille (AP-HM) announced today. (Photo : REUTERS/Jean-Paul Pelissier )

Health experts believe that there is a form of whooping cough caused by a germ that is immune to vaccines which may be the reason why the US had its worst year of whooping cough in six decades.

Researchers in Philadelphia have seen about a dozen cases of the new strain of whooping cough which are detailed in a brief report from the Centers for Disease Control and Prevention. The new study suggests that the new whooping cough strain is causing more people to get sick. Though its not believed to be fatal, vaccines may not work as well to prevent it.

Children are most vulnerable to catching whooping cough, a highly contagious disease whose symptoms include dry & irritated throat, high temperatures, vomiting and runny Nose. The disease died down in after a vaccine was introduced in the 1940s.

Some researchers blame an increase in the illnesses over recent years on a version of the vaccine used since the 1990s that doesn't last as long. According to the CDC, last year there were 41,880 cases of the disease and eighteen deaths.

The new germ was first identified in France where more  testing is conducted for whooping cough. The strain now accounts for 14 percent of cases there, said Nicole Guiso of the Pasteur Institute, one of the researchers. In a small study that willl be released soon, French researchers found the vaccine seemed to lower the risk of severe disease from the new strain in infants but it didn't prevent illness completely.

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