Half of all Americans have some sort of vision problem, mostly myopia or astigmatism, U.S. researchers reported on Monday.

This is far higher than previous estimates, the team at the National Eye Institute reported in the Archives of Ophthalmology.

“Clinically important refractive error affects half of the U.S. population 20 years or older,” wrote Susan Vitale and colleagues at the institute, one of the U.S. government’s National Institutes of Health.

They analyzed data from the National Health and Nutrition Examination Survey on 12,000 people aged 20 and older between 1999 and 2004.

More than 33 percent were nearsighted and 36 percent had astigmatism, which causes fuzzy vision, the team reported. Another 3.6 percent were farsighted, meaning they can see at a distance but not up close.

“Our estimated prevalence of myopia was higher than the 25 percent reported in previous U.S. studies and similar (in persons under 40 years) to that of ethnic Chinese persons in Singapore,” they wrote.

“The direct annual cost of refractive correction for distance visual impairment is estimated to be between $3.8 and $7.2 billion for persons 12 years and older.”

The study matches findings in other countries that have shown about half the population has a so-called refractive vision problem, usually requiring the use of glasses, contact lenses or corrective surgery.

The cause of these three eye conditions is unclear but there is a genetic component. Most studies discount the widely held belief that myopia is caused by too much reading or close work as a child.

WASHINGTON (Reuters)