Whooping cough may be rising to epidemic levels, say the Centers for Diseases Control and Prevention (CDC).
According to the CDC, there have been more than 18,000 cases of whooping cough in the US so far in 2012 - double the number of last year at this time.
"We would need to go back to 1959 to find as many cases reported by this time in the year," said Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, reported WebMd.
The likely cause of the the outbreak is not entirely clear.
According to WebMd, researchers say that some of the possible causes could be better reporting of the disease, its cyclical nature or due to a switch to a new vaccine in the late 1990s that might not attack the current strain as well.
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MedPage Today said that the outbreak in the US, Canada and Australia throws the effectiveness of the vaccine into question.
Since the rate of whooping cough is high in 13 and 14 year olds, scientists suspect that maybe the vaccine is wearing off too early.
"The switch we made ... from whole-cell pertussis to acellular pertussis in 1997 may impact how long vaccination lasts," said Schuchat, reported MedPage Today.
Washington State has been hardest hit by the outbreak, even declaring an epidemic in early April.
The report on the rise of the illness was noted in this week's Morbidity & Mortality Weekly Report by the CDC.