Nurses Fatima Guillen, left, and Fran Wendt, right, give Kimberly Magdeleno, 4, a Tdap whooping cough booster shot as she is held by her mother, Claudia Solorio, at a health clinic in Tacoma, Wash., in a May 2012 file photo. AP Photo/Ted S. Warren

By MIKE STOBBE, AP Medical Writer

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 say 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.

The good news: Despite the high number of illnesses, deaths didn't increase. Eighteen people died, including 15 infants younger than 1.

Officials aren't sure why there weren't more deaths, but think that the attention paid to bad outbreaks across the nation resulted in infected children getting diagnosed faster and treated with antibiotics.

Also, a push last year to vaccinate pregnant women -- a measure designed to pass immunity to infants -- may have had some small measure of success, Clark said.

The final tally will be higher but unlikely to surpass the nearly 63,000 illnesses in 1955, he said.

Whooping cough is a highly contagious disease that can strike people of any age but is most dangerous to children. Its name comes from the sound children make as they gasp for breath.

It used to be a common threat, with hundreds of thousands of cases annually. Cases gradually dropped after a vaccine was introduced in the 1940s.

For about 25 years, fewer than 5,000 cases were reported annually in the U.S. But case counts started to climb again in the 1990s although not every year. Numbers jumped to more than 27,000 in 2010, the year California saw an especially bad epidemic.

Experts looking for an explanation have increasingly looked at a new vaccine introduced in the 1990s, and concluded its protection is not as long-lasting as was previously thought.

Children are routinely vaccinated with five doses beginning at 2 months, and a booster shot is recommended at around 11 or 12. Health officials are considering recommending another booster shot, strengthening the vaccine or devising a new one.