The director of Missouri’s Department of Health and Senior Services granted an exemption Thursday that allows pregnant women and parents of young children to choose whether to receive H1N1 vaccines containing a mercury-based preservative.

A statute prohibits pregnant women and children under 3 from receiving vaccines with the preservative, but Margaret Donnelly determined that a shortage of preservative-free vaccine was preventing those groups from getting the new H1N1, or swine flu, vaccine.

“We know that pregnant women and young children are the most susceptible to this illness,” Donnelly said in a statement. “But delays in vaccine production have created a situation where the most vulnerable people were left without vaccine protection.”