More than 4,300 Minnesota residents were diagnosed with whooping cough this year.
A quarter of the cases were among children ages 9-12. The number is far more than in any year since World War II.
Experts say the reason for the increased number is a redesigned vaccine that was introduced in 1991 to reduce the side effects.
Health experts say the vaccine's protection doesn't last as long, something that has taken time to observe.
Before the first pertussis vaccine became available in the United States in 1943, Minnesota had as many as 5,000 cases some years.