The FDA has received reports of offers to sell unlicensed influenza vaccine in the United States, and of unlicensed individuals administering questionable vaccine to take advantage of reports that flu vaccine is in short supply.

The agency is aggressively working with state health authorities and the Centers for Disease Control and Prevention in Atlanta to investigate questionable vaccine being made available through unusual suppliers, the FDA said in a statement.

The FDA has licensed three influenza vaccines for use: Fluzone, manufactured by Aventis Pasteur; Fluvirin, manufactured by Evans Vaccines -- both of which are given as injections; and FluMist, manufactured by Medimmune, which is given as a nasal spray. These vaccines are sold from licensed sources, such as doctors, pharmacies and health care clinics.

"Careful discretion should be used in determining whether the vaccine is being sold by reputable distributors with proper evidence that the vaccine is licensed for sale in the United States," the FDA said.

"Consumers should obtain influenza vaccine from licensed health care providers only."