Every day, a school bus drops off as many as 45 children at a community eye clinic on Chicago’s South Side. Many of them are referred to the clinic after failing vision screenings at their public schools.

Clinicians and students from the Illinois College of Optometry give the children comprehensive eye exams, which feature refraction tests to determine a correct prescription for eyeglasses and dilation of their pupils to examine their eyes, including the optic nerve and retina.

No family pays out-of-pocket for the exam. The program bills insurance if the children have coverage, but about a third are uninsured. Operated in partnership with Chicago public schools, the program annually serves up to 7,000 children from birth through high school.

“Many of the kids we’re serving fall through the cracks,” said Dr. Sandra Block, a professor of optometry at the Illinois College of Optometry and medical director of the school-based vision clinics program. Many are low-income Hispanic and African-American children whose parents may not speak English or are immigrants who are not in the country legally.

Falling through the cracks is not an uncommon problem when it comes to vision care. According to a 2016 report from the National Academies of Sciences, Engineering and Medicine, as many as 16 million people in the United States have undiagnosed or uncorrected “refractive” errors that could be fixed with eyeglasses, contact lenses or surgery. And while insurance coverage for eye exams and corrective lenses clearly has improved, significant gaps remain.

The national academies’ report noted that impaired vision affects how people experience their world, including normal communication and social activities, independence and mobility. Not seeing clearly can hamper children’s academic achievement, social development and long-term health.

But when people must choose, vision care may lose out to more pressing medical concerns, said Block, who was on the committee that developed the report.

“Vision issues are not life-threatening,” she said. “People get through their day knowing they can’t see as well as they’d like.”

Insurance can make regular eye exams, glasses and treatment for medical problems such as cataracts more accessible and affordable. But comprehensive vision coverage is often achieved only through a patchwork of plans.

The Medicare program that provides coverage for millions of Americans age 65 and older doesn’t include routine eye exams, refraction testing or eyeglasses. Some tests are covered if you’re at high risk for a condition such as glaucoma, for example. And if you develop a vision-related medical condition such as cataracts, the program will cover your medical care.

But if you’re just a normal 70-year-old and you want to get your eyes examined, the program won’t cover it, said Dr. David Glasser, an ophthalmologist in Columbia, Md., who is a clinical spokesman for the American Academy of Ophthalmology. Many commercial health insurance plans also exclude routine vision care from their coverage. Employers may offer workers a separate vision plan to fill in the gaps.

Vision coverage for kids improved under the Affordable Care Act. The law requires most plans sold on the individual and small-group market to offer vision benefits for children younger than 19. That generally means that those plans cover a comprehensive eye exam, including refraction, every year, as well as a pair of glasses or contact lenses.

But since pediatric eye exams aren’t considered preventive care that must be covered without charging people anything out-of-pocket under the ACA, they’re subject to copays and the deductible.

Medicaid programs for low-income people also typically cover vision benefits for children and sometimes for adults as well, said Dr. Christopher Quinn, president of the American Optometric Association, a professional group.

But coverage alone isn’t enough. To bring down the number of people with undiagnosed or uncorrected vision, education is key to helping people understand the importance of eye health in maintaining good vision. Just as important, it can also reduce the impact of chronic conditions such as diabetes, the national academies’ report found.

“All health care providers need to at least ask vision questions when providing primary care,” said Block.