Fri, May 1, 2009 (HealthDay News) -- The number of confirmed swine flu cases in the United States has surpassed 100, federal health officials said Thursday, but the infections continue to be mild.

In response to the outbreak, scientists are racing to produce a vaccine for all Americans against the never-before-seen flu strain, but the shots -- if needed at all -- wouldn't be available until fall at the earliest, federal health officials said.

"We think 600 million doses is achievable in a six-month time frame" from that fall start, Health and Human Services Assistant Secretary Craig Vanderwagen told lawmakers Thursday.

Meanwhile, the news out of Mexico -- believed to be the source of the outbreak -- offered some hope late Thursday as Health Secretary Jose Angel Cordova said the number of new swine flu cases seemed to be leveling off.

"The fact that we have a stabilization in the daily numbers, even a drop, makes us optimistic," Cordova said at a news conference. "Because what we'd expect is geometric or exponential growth. And that hasn't been the situation."

Still, Mexico braced for a shutdown of all non-essential services through Tuesday as authorities sought to limit further infections in that country, where the virus is suspected of causing 168 deaths so far.

In the United States, at a Thursday morning news conference, Dr. Richard Besser, acting head of the U.S. Centers for Disease Control and Prevention, said, "Today I am reporting 109 confirmed cases within the United States. We have 11 affected states. There are many more states that have suspect cases," he added.

There are 50 confirmed cases in New York, 26 in Texas, 14 in California, 10 in South Carolina, two each in Kansas and Massachusetts and one each in Arizona, Indiana, Michigan, Nevada, and Ohio, Besser said. The age range of those infected is 22 months to 81 years, he said.

"Six of the cases have been hospitalized, including the unfortunate case we reported yesterday of the child in Texas who passed away," he said.

The flu strain is a combination of pig, bird and human viruses, prompting worries from health officials that humans may have no natural immunity to the pathogen.

Besser said federal health officials "continue to be very aggressive in our approach and we will continue to do that until the situation tells us that we no longer need to do so. There's no one action that is going to stop this. There is no silver bullet, but all the efforts -- the efforts of government, the efforts of communities and the efforts of individuals -- will help to reduce the impact on people's health."

"There are things people can do," he said, including "handwashing, covering coughs, staying at home when they are sick.

The vaccine plan is to complete the production of seasonal flu vaccine for next winter and then switch to production of a vaccine for the N1H1 swine flu, if needed, Besser said.

All of the cases diagnosed in the United States continue to be mild, federal health officials said Thursday.

The swine flu outbreak has even touched the White House -- an aide to Energy Secretary Steven Chu apparently got sick while helping arrange President Barack Obama's recent trip to Mexico. The aide did not fly on Air Force One, however, and never posed a risk to the president, the Associated Press reported.

On Wednesday, President Obama said that U.S. public health officials were recommending that schools with confirmed or suspected cases of swine flu "should strongly consider temporarily closing so that we can be as safe as possible."

Nearly 300 schools have been closed nationwide in response to the outbreak, according to news reports.

Texas has postponed all public high school sports and academic competitions at least until May 11 due to the outbreak.

On Thursday, Fort Worth, Texas, announced the temporarily closure of all district schools for its approximately 80,000 students, probably until May 12, after one student was found to be infected with swine flu and three others were suspected of suffering from the virus.

As with the previously tested strains of the swine flu virus, new testing has found that the pathogen remains susceptible to the two common antiviral drugs Tamiflu and Relenza, according to an April 28 dispatch from the CDC's Morbidity and Mortality Weekly Report.

On Wednesday, the World Health Organization raised the swine flu epidemic level from 4 to 5, signifying that a pandemic is imminent, and urged countries to implement their pandemic plans. And on Thursday, WHO said it would cease calling the virus "swine flu," using instead its scientific name -- H1N1 influenza A -- to help reduce confusion over the danger posed by pigs. Pork consumption does not transmit infection.

Meanwhile, residents of Mexico prepared Thursday for a broad shutdown of services as officials urged businesses to close until Tuesday, to coincide with a long holiday weekend. Mexican President Felipe Calderon said in a televised address that only essential businesses such as supermarkets, hospitals and pharmacies should stay open, and only critical government workers such as police and soldiers would be on duty from Friday through Tuesday. School had already been canceled nationwide through Tuesday, The New York Times and the AP reported.

On Monday, a 23-month-old Mexican boy who had traveled to Houston for medical treatment died, becoming the first fatality in the United States.

Switzerland and the Netherlands have become the latest countries to report swine flu infections. In addition to Mexico and the United States, Canada, New Zealand, Britain, Germany, Spain, Israel and Austria also have confirmed cases, the AP reported.