whooping_cough_web.jpgATLANTA (AP) — More than 21,000 people got whooping cough last year, many of them children and teens. That's the highest number since 2005 and among the worst years in more than half a century, U.S. health officials said recently.

Officials are puzzled by the sharp rise in cases. The vaccine against whooping cough is highly effective in children and vaccination rates for kids are good.

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.

California appeared to be the hardest-hit state last year, with state health officials reporting more than 8,300 cases, including the deaths of 10 babies.

Nationally, there were at least 26 deaths, the Centers for Disease Control and Prevention said.

The national case count is preliminary and may wind up being higher. The numbers were reported at a recent vaccine advisory committee meeting.

Health officials believe contagious adolescents are a worrisome threat to vulnerable infants. About 95 percent of children have had at least three shots against whooping cough. But because a whooping cough vaccine for adolescents and adults was not licensed until 2005, vaccination rates for those groups are much lower. One study estimated that only six percent of adults are fully immunized.

The Advisory Committee on Immunization Practices recommends the whooping cough vaccine for all adults who are around infants. The committee voted to slightly alter vaccination guidance to make it clear that all nurses and other health care workers should get the vaccine.