(05-01) 04:00 PDT Mexico City - --

Mexican health authorities said Thursday that they have confirmed 300 swine flu cases, as the World Health Organization cautioned there is no evidence the worst of the global outbreak is over.

Health Secretary Jose Cordova said authorities have spent $116 million on medical supplies and equipment so far in the epidemic, which has caused a confirmed 12 deaths in Mexico.

The U.S. caseload rose slightly to at least 130 as hundreds of schools nationwide shut their doors, and the crisis even reached the White House, which said an aide to the secretary of energy apparently got sick helping arrange a presidential trip to Mexico.

European health ministers vowed to work quickly with drugmakers to produce a vaccine, but American health officials suggested inoculations could not begin until fall at the earliest.

Earlier Thursday, Cordova had expressed some optimism, saying new cases have leveled off and the death rate has been nearly flat for several days. He said the next few days would be critical in determining whether the virus was truly on the decline.

"The fact that we have a stabilization in the daily numbers, even a drop, makes us optimistic," he said. "Because what we'd expect is geometric or exponential growth. And that hasn't been the situation. So we think we're on the right track."

The World Health Organization's top flu official sounded a more cautious note about whether the virus had peaked.

"For things to go up and down in a country is expected. If it didn't do that would be very unusual," Dr. Keiji Fukuda said in Geneva. "Hopefully, we'll see more of the data of what's going on there. But I expect even in Mexico you will see a mixed picture."

Mexico has imposed what amounts to a five-day shutdown of the country, beginning today, in hopes of slowing the virus. All but the most essential government services will be suspended, most businesses have been urged to close, and Mexicans have been encouraged to stay in their homes.

In other news, an aide to Energy Secretary Steven Chu apparently got sick helping arrange President Obama's recent trip to Mexico. The aide did not fly on Air Force One and never posed a risk to the president, the White House said.

The United States is taking extraordinary precautions, including shipping millions of doses of anti-flu drugs to states. Scientists cannot predict what a new virus might do, and the outbreak could always resurge later.