A new law in California is expected to make it more difficult for parents to avoid vaccinating their children who plan to attend school.

The move is expected to shore up vaccination rates within a state that has been among the most effective over the past decade at bolstering immunizations among kindergartners.

The law will take effect Jan. 1 and is designed to make it more difficult for parents to obtain illegitimate medical exemptions that allow children to avoid vaccinations. Medical exemptions are allowed for children with compromised immune systems, and who would not be able to safely receive certain vaccines.

Under the new law, California’s Department of Public Health will make the final call on which children are granted exemptions, closing a vaccination loophole and upsetting an anti-vaxxer community that believes immunizations may be detrimental to children’s long-term health, despite scientific evidence to the contrary.