More than 21,000 people had whopping cough in 2010 – the highest amount since 2005 – and among the worst reported in more than 50 years, federal health officials reported.

Many of the infected were children and teens.

An advisory committee reported the numbers at a meeting in Atlanta. Health experts are puzzled by the spike in numbers since the whopping cough vaccine is considered highly effecting in children – and most children receive the vaccine.

The disease is very contagious and in rare cases can be fatal, especially for babies too young to be vaccinated. Whooping cough starts like a cold but leads to severe coughing that can last for weeks.

The Associated Press contributed to this article.

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