DENVER -- About 4,300 babies born at University Hospital in the past two years need to get the hepatitis B vaccine again because their first shot may have been defective.

The vaccine may have frozen in storage, which could have reduced its effectiveness, hospital spokeswoman Sarah Ellis said Monday.

Ellis said there is no evidence the vaccine was ineffective, but "we just can't assume they're all OK."

The parents will be notified by letter.

Parents will not be charged for revaccination. The cost to the publicly funded hospital could exceed $300,000 because the vaccine costs $75 a shot.

The vaccine is usually given in a series of three shots. Babies who have had more than two additional shots probably do not need to be revaccinated, said Dr. William Letson, a pediatric epidemiologist at the Colorado Department of Public Health and Environment.

However, an extra shot will not harm a child, he said.

The suspect vaccine was given to babies born at University between February 2002 and October 2003. The vaccination would not have harmed the children, hospital officials said.

An inspector from the state health department discovered significant temperature variations in the refrigerator where the vaccine was kept while reviewing records, Letson said.

The variations were first discovered in October but it took several months to determine how many babies were affected and how to alert their parents.

The hepatitis B vaccine, like many others, needs to be kept between 35 and 46 degrees Fahrenheit, Letson said. "Most vaccines won't tolerate freezing," he said.

Hospital officials blamed a defective thermometer and said it had been replaced.

Hepatitis B is caused by a virus that attacks the liver. It can cause lifelong infection, scarring of the liver, liver cancer, liver failure and death.

Parents with questions can call (303) 372-5889 but all affected parents will receive a notice in the mail.