An investigation by the Centers for Disease Control found that free vaccines meant for children as part of a government program may have been stored at the wrong temperature.
The violations are considered serious by inspectors.
Seventy-five percent of the doctors' offices and clinics that were checked had been storing their vaccines at temperatures considered either too hot or too cold. They also had damaged goods in the vaccine refrigerators, and one out of four had expired vaccines.
Experts say, poorly stored vaccines are not deadly or harmful, but they can become ineffective. That can leave children exposed to diseases these drugs are designed to protect them against.
"There have been changes in the equipment, the refrigerators," said CDC's Dr. Anne Schuchat "There are many vaccines recommended now and it maybe there are more doses being stored in the average office than there used to be."
The CDC is vowing to improve the system with retraining and more oversight.
It is recommended parents talk to their doctors about children's vaccines and how they are being stored.