WASHINGTON - Warning that the worst may be yet to come, U.S. officials yesterday declared the spread of swine flu to be a public health emergency and freed up 12.5 million doses of antiviral medication to help fight the disease, which has now infected 20 people in five states.

The move came as state and local authorities, the Centers for Disease Control and Prevention, and the international health community stepped up public awareness, testing, and surveillance to stop the disease before it becomes a global epidemic.

In Mexico, where the outbreak originated, the federal health secretary said late yesterday that the number of suspected cases now tops 1,600, including 103 deaths.

No deaths have been reported in the United States, but officials confirmed yesterday that eight students at a New York City high school tested positive for the disease after dozens had complained of flulike symptoms.

Some of the students had recently returned from a spring break trip to Cancun, Mexico. The school has been closed as a precaution.

Authorities have confirmed seven cases in California, two each in Kansas and Texas and one in Ohio. More cases are expected in the coming days.

Yesterday, four cases were confirmed in the Canadian province of Nova Scotia. New Zealand said 10 students who took a school trip to Mexico "likely" had swine flu, and Spanish authorities had seven suspected cases under observation, according to the Associated Press.

The World Health Organization said the new strain of swine flu has "pandemic potential." A flu pandemic occurs when a new influenza virus emerges, people have little or no immunity to it, and there is no vaccine for it.

"As we continue to look for cases, I expect that we're going to find them. We've ramped up our surveillance around the country to try and understand better what is the scope, what is the magnitude of this outbreak," said Richard Besser, acting director of the CDC.

President Obama is getting regular briefings on the outbreak and the steps being taken to address the problem.

Homeland Security Secretary Janet Napolitano is heading the federal effort to deal with the outbreak, but numerous other departments, including Health and Human Services and Agriculture, are also involved.

The virus spreading throughout the United States appears to be the same strain that has infected Mexico, but health officials were unsure why the U.S. outbreaks were not as severe. To date, all U.S. sufferers have recovered, and only one has been hospitalized.

That could change, however, since infectious diseases, and the flu virus in particular, are unpredictable, Besser said.

"Given the reports out of Mexico, I would expect that over time, we're going to see more severe disease in this country," Besser warned. "We do think that this will continue to spread, but we are taking aggressive actions to minimize the impact on people's health."

Yesterday's emergency declaration frees up federal, state, and local resources for disease prevention. The move allows agencies to conduct diagnostic tests, if necessary, on young children.

It also approves the release of 12.5 million courses of antiviral medications Tamiflu and Relenza from the nation's strategic stockpiles. The drugs will go to states in need, with priority given to those with confirmed cases.

The Department of Defense is making an additional seven million courses of Tamiflu available, Napolitano said.

Both Tamiflu and Relenza help lessen the severity of flu symptoms and shorten the course of the illness, but it is unclear whether the drugs will work against the swine flu, Besser said. Since there is no vaccine for swine flu, health officials are trying to determine whether a vaccine could be developed, Besser said.

In addition, the Agriculture Department is screening and testing livestock to make sure the food supply is not infected, Napolitano said. She said the disease cannot be contracted from eating properly cooked pork.

Travelers entering the United States from infected locations will be subject to "passive surveillance," in which they will be asked questions about their health. Those with symptoms will be isolated, given protective equipment, and could be subject to further testing, Napolitano said.

Although the State Department has not issued an official travel advisory for Mexico, that could change.

"Right now, we don't think the facts warrant a more active testing or screening of passengers coming in from Mexico," Napolitano said.

She urged travelers to check the State Department's Web site for updates.

Updates, details and tips for preventing disease at http://go.philly.com/swineflu EndText