* New U.S. MedImmune vaccine order brings total to 42 mln
* MedImmune says U.S. contract worth $453 mln
* Says vaccine already been manufactured
NEW YORK, Sept 21 AstraZeneca's (AZN.L) MedImmune unit said on Monday that the U.S. government has ordered an additional 29 million doses of its H1N1 swine flu vaccine.
The vaccine, in nasal spray form, was approved by U.S. health regulators last week for use in people aged two to 49 against the strain of swine flu that is very easily spread among humans and has been declared a pandemic.
The U.S. Department of Health and Human Services (HHS) had previously ordered 13 million doses of the MedImmune vaccine, bringing the total to 42 million doses at a cost of about $453 million, the company said.
Enough bulk vaccine to fill all orders placed by HHS has already been manufactured and about 3.4 million doses have been released by the U.S. Food and Drug Administration, MedImmune said.
"We will continue to work collaboratively with the U.S. government and public health authorities to contribute to the response to this unpredictable infectious disease," MedImmune President Tony Zook said in a statement.
The United States has now ordered some 251 million doses of H1N1 vaccine from five manufacturers -- not enough to cover the entire U.S. population of 300 million people, especially with some patients expected to require two doses. (Reporting by Bill Berkrot; editing by Carol Bishopric)