Some people may develop a potentially dangerous eye problem due to the shingles vaccine, a new study finds.

The varicella zoster virus vaccine was designed to protect against chickenpox in children and shingles in adults, and has been in use for some 20 years. But now researchers at the University of Missouri School of Medicine say it may cause  keratitis, an eye inflammation, in a rare number of cases.

Keratitis, or inflammation of the clear layer on the front of the eye, is a vision issue that can cause serious complications or even permanent damage to vision if left untreated. This eye inflammation can be caused by infection, bacteria, and the virus that causes shingles.

By studying case reports from national and international registries, the researchers discovered at least 20 cases of keratitis occurred in children and adults within a month of administration of the chickenpox and shingles vaccine.

The majority of children and adults should still receive the vaccination, however, those with a history of this eye problem should discuss the issue with their doctor so that they can be properly monitored afterwards, said the researchers at the 2015 meeting of the American Academy of Ophthalmologists in Las Vegas.