Under no circumstances should the border be closed in response to the swine flu outbreak, a Centers for Disease Control (CDC) official told Congress today.

Testifying before the Senate Homeland Security Committee, Rear Admiral Anne Schuchat said the a border closure would not be effective in preventing the spread of the swine flu.

Pressed by Sen. John McCain (R-Ariz.) on what what conditions would merit closing the border, Schuchat said none would.

"I don't think there are any," she replied.

Senators expressed shock at Schuchat's statement.

"I'm surprised at your answer, Admiral," said Sen. Joe Lieberman (I-Conn.), adding that he "hope[s] we'll keep open the possibility that we might for some period of time close the ports of entry."

McCain urged Schuchat and Homeland Security Secretary Janet Napolitano, who was also testifying, to "keep revisiting this issue of whether we should close the border or not."

Napolitano opposed closing the border, but seemed not to rule it out as a future possibility.

"Making such a [border] closure right now has not been merited by the facts," Napolitano said.

Senators also urged the administration to step up observation of travelers at the border, especially the use of thermal scanners. Both Napolitano and Schuchat said thermal scanning was ineffective, since some flu victims did not present with a fever.

"Even if that technology isn't perfect...it seems to me using technology to try to identity some of the individuals would make sense," said Sen. Susan Collins (R-Maine.)