Experts say the reason for the high number is a redesigned vaccine that was introduced in 1991 to reduce the side effects.

The whooping cough hit Minnesota residents hard this year — with the biggest number of cases since World War II.
More than 4,300 Minnesota residents had confirmed, probable or suspected cases of the respiratory infection called pertussis this year, according to Minnesota Public Radio News. There were 661 in 2011. The numbers reflect a national increase in cases.
"I can tell you that our epidemiologists are exhausted," said Kris Ehresmann, director of Infectious Disease Epidemiology at the Minnesota Department of Health. "We're getting 20-plus reports a day of pertussis."
Experts say the reason for the high number is a redesigned vaccine that was introduced in 1991 to reduce the side effects.
Patsy Stinchfield, director of pediatric infectious disease services for Children's Hospitals and Clinics of Minnesota, said the vaccine's protection doesn't last as long.
"It's evolutionary," Stinchfield said. "You have to sort of watch how it evolves. You make a change to a vaccine in the '90s and you evaluate it a decade later."
Immunity begins to wane around three years after vaccination, Ehresmann said. So by the time fully immunized kids reach adolescence, they have very little protection left.
"My hope is that with some changes in the immunization recommendations that this won't continue to be the new normal," Ehresmann said.
There is a booster vaccine available for people aged 10 years and older.
Before the first pertussis vaccine became available in the United States in 1943, Minnesota had as many as 5,000 cases some years.
One of the Minnesotans affected in 2012 was now 3-month-old Heather Chasse.
Her mother, Rachel, said she took her daughter into the clinic with a severe cough when she was 2-weeks old. Rachel was waiting in an exam room when her daughter had a particularly bad coughing episode and had trouble breathing.
"She turned dark, dark red-purple and went limp in my arms, and her eyes got really big, and she just stopped making noise," said Rachel Chasse. "She just stopped doing any of this and I started screaming,".
The baby was taken by ambulance to Children's Hospital in Minneapolis, where she spent two weeks. At one point her oxygen levels plunged as low as 17 percent. A normal reading is around 100 percent.
The baby is much better now but when she gets agitated, there are still remnants of her pertussis cough.