Mass clinics for mumps immunization will open for young adults next week in Iowa, the state at the center of a regional epidemic, health officials said Thursday.

The state will target 18- to 22-year-olds, dividing 25,000 doses among counties with colleges, universities and post-secondary institutions, where students are especially vulnerable to contracting and spreading the virus, said Mary Mincer Hansen, Iowa public health director.

The immunization plan reflects a change from earlier statements by state public health officials that the outbreak of the annoying but rarely serious disease was no reason to cancel events or start widespread immunization.

The mumps outbreak is being called the nation's worst in 20 years. As of Thursday, Iowa had 975 cases of probable, confirmed and suspected cases, said Dr. Patricia Quinlisk, the state epidemiologist.

Cases have been reported in at least seven other states.

Mumps is spread by coughing and sneezing. Common symptoms are fever, headache and swollen glands under the jaw. It can lead to problems including hearing loss, meningitis and swollen testicles, which can lead to infertility. It does not respond to antibiotics.