"I was very pleasantly surprised," Dr. Jeffrey Engel said, noting none of the 15 even tested positive for a seasonal flu. "The situation appears to be stable."
A few dozen samples still need to be tested in the coming days, Engel said, and more suspect cases continue to turn up around the state.
"These things change on an hour-to-hour basis," he said. "My guess is this is going to be ongoing for weeks to months. We're in for a long haul."
One of the negative tests was for a traveler who got off a plane from Mexico in Charlotte and went to a hospital because of flu-like symptoms.
One other traveler also went to a Charlotte hospital this week with flu-like symptoms, Engel said. Tests are not complete on that person, who was about to return home to Texas but decided to see an emergency room physician.
Engel urged calm Tuesday as officials continue trying to determine if there are any "probable" cases of swine flu in North Carolina. People suspected of having the disease have been ordered to remain in isolation at home.
"We're treating suspects as if they have the disease," he said. "That's the best way to control it."
The state has resisted issuing any quarantine orders for healthy people who have been in contact with suspect cases, he said, noting that educating households about hygiene and how to maintain a proper distance from an ill person is enough at this point to keep the disease in check.
"We want science to be driving all of the policy," he said during a news conference. "We're not going to be running this issue with fear, but with scientific facts."
Where the cases of swine flu are
Across the country, health officials had identified 64 cases of swine flu – most in New York City – as of Tuesday evening. Of those, five have been hospitalized and none has died.
The global health agency says, so far, most people confirmed with swine flu were in Mexico. In that country, the swine flu strain is suspected in more than 150 deaths, and more than 1,000 others may be suffering symptoms.
The Centers for Disease Control and Prevention reported that 85 percent of the confirmed U.S. cases weren't associated with travel, according to Engel.
Yet, Cuba on Tuesday banned travel to Mexico. Several European countries also issued travel advisories.
Scotland and Spain reported confirmed cases of swine flu on Tuesday.
On Monday, Engel said the state is "better prepared" than ever to handle any potential outbreak of the virus in North Carolina. The state has a stockpile of 660,000 doses of treatment for the flu, if necessary.
North Carolina also will receive next week a quarter of its allotment from a national strategic stockpile of medications and masks, he said. But the commercial marketplace had more than enough supply of anti-viral drugs to meet state demand, he said.
Engel urged residents to take precautions by staying at home if they have flu-like symptoms, washing their hands frequently with soap and water for at least 15 seconds and seeing a doctor if flu symptoms become serious.
"To ward off any potential, just be prudent as you would during normal flu season," he said.
President Barack Obama on Tuesday asked Congress to approve $1.5 billion to fight the outbreak. Money would go toward improved monitoring to identify and isolate suspected cases and upgrades in testing both to confirm cases and to develop vaccines.