The Obama administration is standing by its decision to keep the U.S. border with Mexico open - despite the swine flu outbreak.

U.S. Vice President Joe Biden spoke Thursday in interviews on CBS and NBC television.  He said closing the border would be a "monumental undertaking" with limited benefits.  

However, Biden said he would advise against members of his own family flying on airplanes or traveling by train.  He said he worries about the risk of infection in confined spaces.

But the White House sought to clarify that remark, saying the advice was in line with what the administration has been saying - that Americans should avoid unnecessary air travel to and from Mexico, and that if a person is sick, he or she should avoid airplanes and other confined public spaces such as subways.

Meanwhile, Mexican President Felipe Calderon is urging people to stay at home while the federal government suspends non-essential activities in the country, where concerns about the flu's impact on the economy are growing.  

Many countries are urging citizens to avoid all non-essential travel to Mexico.  Mexico's Treasury Minister Agustin Carstens says the outbreak will cost Mexico between 0.3 and 0.5 percent of its gross domestic product.

The U.S. Centers for Disease Control and Prevention says at least 91 people in 10 U.S. states have been infected.  

CDC Acting Director Dr. Richard Besser says the U.S. expects the number of severe cases and deaths from this strain of the flu to rise.  He also says the CDC is evaluating whether school closures are helping stop the spread of the virus.

Some schools in the United States have already shut down.  The city of Fort Worth, Texas says all 144 of its schools will be closed indefinitely beginning Thursday, after a case of swine flu was confirmed at one school.  

U.S. Health and Human Services Secretary Kathleen Sebelius says the federal government has 50 million courses of treatment of the antiviral drugs Tamiflu and Relenza, used to treat swine flu, and has begun shipping portions of the stockpiles to states.