States that make it easier for parents to refuse vaccination for their children may put local kids with legitimate medical reasons to avoid vaccines at greater risk of infection, according to a new study published today (August 30) in The Journal of Infectious Diseases. As the percentage of the vaccinated population falls below a critical threshold, the area is no longer protected by the phenomenon of herd immunity, in which the entire community is shielded from disease as a result of too few susceptible hosts for the pathogen.

Conducted between 2004 and 2011, the new study looked at more than 87,500 medical exemptions nationwide and found that parents were more likely to refuse vaccination in states with easier requirements. “Medical providers, parents, school officials, and state health officials are responsible for ensuring that medical exemptions are actually medically indicated,” said senior author Saad Omer in a press release.

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