According to an Exxon Mobil spokeswoman, it was the company's first use of an outside contractor to administer them, adding the FBI has told them it was "definitely not the flu vaccine."
A prosecutor says it does not appear the fake shots are harmful, but that steps are being taken to ensure workers' safety.
Exxon Mobil has offered blood tests and counseling to about 1,000 employees who received shots during the health fair.
The FBI and the Food and Drug Administration are investigating the contents of the syringes and whether others might have received the fake vaccine.
