Researchers have come across 20 cases recently in children and adults that occurred soon after vaccination. The adults developed keratitis within 24 days of vaccination, while children started suffering within 14 days.

People who have a history of keratitis or other inflammatory eye problems should tell their doctor before they have the vaccination, say researchers from the University of Missouri-Columbia, who made the discovery. They fear the vaccine—the varicella zoster virus vaccine for chickenpox and shingles—could trigger new inflammatory problems.

The researchers say the reaction is rare, but they do not know the true extent of the problem as they have had to rely on voluntarily reported cases gathered from the US Food and Drug Administration and the World Health Organization adverse events databases.