The nation just suffered its worst year for whooping cough in nearly six decades, according to preliminary government figures.

Whooping cough ebbs and flows in multi-year cycles, and experts said 2012 appears to have reached a peak with 41,880 cases. Another factor: A vaccine used since the 90s doesn’t last as long as the old one.

The vaccine problem may continue to cause higher than normal case counts in the future, said Dr. Tom Clark of the Centers for Disease Control and Prevention.

“I think the numbers are going to trend up,” he said. The agency provided the latest figures on Friday.

Last year, cases were up in 48 states and outbreaks were particularly bad in Colorado, Minnesota, Washington state, Wisconsin and Vermont.