18,700 of the University of Virginia's (UVA) 21,000 student had their Social Security numbers exposed in a mailing gaffe, the Daily Progress reported.

Aetna Health Care, the school's provider of health insurance, sent brochures to students' homes across the country with their Social Security number printed on the address label. The brochures were sent using a third-party mail carrier.

"It's definitely easily visible if you know what you're looking for," said Elliott, an editor at the school newspaper, the Cavalier Daily, which broke the story Wednesday afternoon. "It isn't separated by the little dashes, but it still looks like a Social Security number."

School officials said they learned of the error last week and were trying to determine if Aetna or anyone from the university met with the mailers before sending the brochures.

"The university certainly regrets that this exposure occurred," university spokesman McGregor McCance said in an email. "The university is in the process of notifying students whose information was exposed."

Aetna spokeswoman Cynthia Michener said the company learned of the mistake earlier in the week.

"We are working with UVa helping to notify the students," Michener said, including she could not disclose further details. "We are trying to do the right thing."

Aetna identified the problem and said the mail carrier did not follow protocol, but that the healthcare provider deserved some blame.

"Aetna's standard protocol with the vendor is to review samples of the mail before a mailing goes out. That procedure was not followed by the vendor in this circumstance," said Michener. "However, this mail vendor does business for Aetna, and as such, we share the responsibility for this mailing."

The letters were most likely sorted electronically and were only reviewed by one or two people.

"They are pre-sorted, then it goes to a certain route, then directly to the carrier," U.S. Postal Service spokeswoman Freda Sauter told the Roanoke Times. "It's not like 20 people would have seen each piece."

Students were unaware of the error and some only found out a week after the error was found.

"I checked my emails this morning and did not see anything about it, but this isn't good," third-year student Ana Turenkov said as she left class Thursday afternoon. "If it had happened yesterday, I feel like they should have said something right away."