Ontario's privacy commissioner is investigating the loss of medical records of thousands people in Ontario's Durham region after a nurse misplaced a USB key.

The device contained data collected from more than 83,000 patients during flu vaccination clinics in the region between Oct. 23 and Dec. 15.

A health department nurse lost the key at Durham region's headquarters in Whitby, Ont, officials said.

The privacy commission office was told of the loss Monday, said spokesman Bob Spence.

The focus of the investigation will be on what happened, how it happened and what steps can be taken to prevent a similar incident from occurring in the future, Spence said.

The health department said in a statement that although vital personal information was on the key, but "the ability to read the personal health information is limited."

The files found on the USB key contain a "lot of gobbledegook, and then some information that is clearly legible," Dr. Robert Kyle, Durham Region's Medical Officer of Health, told CBC News.

" The information that's clearly legible... is name, address, phone number, health card [numbers.]"

There is no reason the device was taken deliberately, health officials said, but added authorities are conducting a "thorough search" for the key.

"The Health Department sincerely apologizes for this lapse in protection and safekeeping of personal health information," Durham region said in a statement.

With files from The Canadian Press