May 6 -- Production of hundreds of millions of doses of swine flu vaccine may move ahead next week after the virus spread to 23 countries and 41 U.S. states, world health officials said.

The number of confirmed cases in the U.S. jumped to at least 642, led by an outbreak in Illinois, after state health labs received kits to perform their own tests of people with symptoms, the U.S. Centers for Disease control and Prevention said today on its Web site. The cases include two U.S. deaths and may represent a fraction of the total cases, officials said.

Disease trackers are monitoring 81 cases in Spain and 27 in the U.K. to determine whether the virus formally called H1N1 has established itself outside North America. Such a finding would prompt the World Health Organization to declare a pandemic, the first since 1968, the agency said. A WHO panel will meet next week to discuss whether vaccine makers should begin producing a swine flu shot once they finish with the seasonal flu vaccine.

"We have recommended for all manufacturers to put everything into place to be able to start manufacturing the vaccines," Marie-Paule Kieny, director of WHO's initiative on vaccine research, said today at a news conference in Geneva. "A further review might be potentially to stop seasonal vaccine production."

WHO, the United Nations' health agency based in Geneva, determined that a swine flu shot would have to be made in separate plants from the seasonal flu version. Swine flu vaccine may also require a follow-up booster shot in order to be effective because it is an entirely new strain, Kieny said.

The single-shot seasonal flu vaccine itself acts as a booster, reinforcing natural antibodies from previous exposures to viruses.

Cases Mount

Swine flu has killed two people in the U.S. and 42 in Mexico. Illinois health officials today reported 227 cases, not all of which were included in the CDC's most recent tally, and said most of the jump is attributable to the state's new testing capability. Before this week, only the CDC lab in Atlanta could definitively identify U.S. cases of swine flu. Test kits were delivered yesterday to laboratories in all 50 states.

Countries that haven't seen cases should prepare for outbreaks, Richard Besser, acting director of the CDC, said today. The agency yesterday also reversed U.S. school-closure recommendations that shut 468,000 students out of classes, saying schools should reopen and sick children should stay home.

Mexico's First Reports

The CDC said Mexico's initial reports that the virus was disproportionately attacking and killing young people led to the agency's advice that schools close if they suspected anyone sick with swine flu. Yesterday's reversal was based on research showing the virus causes less severe symptoms than originally feared, Besser said. Also, the virus has already rooted itself in communities across the country, making containment impossible, he said.

Swine flu has been confirmed by laboratory tests in 1,658 patients in 23 countries, including 30 who died, according to the WHO. Mexico reported 1,112 cases, including 42 deaths. Canada has 165 cases, WHO said. Sweden today became the 23rd country to confirm a case of the illness.

Data so far suggest that the virus affects youth more than seasonal influenza, and that younger patients are entering hospitals, Besser said today. Few with swine flu are older than 60, and the median age is 16. It's possible that older people have greater immunity or that younger people spread the disease on spring break vacation trips to Mexico, he said.

Spreading Threat

Even if symptoms are mild, the ease with which the new virus spreads makes it a threat, according to the CDC.

Both U.S. deaths have been in Texas. The first was a 22- month-old infant from Mexico taken to a Houston hospital, where he died April 27. The second was a 33-year-old school teacher from the area of Harlington, Texas, who contracted the illness in the U.S. and died yesterday, Cameron County health officials said.

The three main seasonal flu strains -- H3N2, another form of H1N1, and type B -- cause 250,000 to 500,000 deaths a year globally, according to the WHO. The new flu's symptoms are similar: aches, coughing, and fever.

Mexican Health Minister Jose Cordova said his country is seeing fewer cases of swine flu, although it's too early to say the outbreak is over.

"In the past few days we've seen a big" decline in new cases, Cordova said yesterday in an interview. "We can't say that it's under control, because ‘under control' is when you don't have any new cases."

Open for Business

Restaurants in Mexico City and businesses in most areas of the country are allowed to reopen today and universities and high schools can resume tomorrow. Government offices that had been shut by the flu outbreak also will open today. Elementary schools will start May 11.

In addition to the North American countries, swine flu has been confirmed in Austria, Colombia, Costa Rica, Denmark, El Salvador, France, Germany, Guatemala, China (Hong Kong), Ireland, Israel, Italy, the Netherlands, New Zealand, Portugal, South Korea, Spain, Sweden, Switzerland and the U.K.

The virulence of the swine flu may reveal itself when the Southern Hemisphere faces its influenza season beginning this month through September, Besser said. Scientists will watch the virus to see whether it becomes the dominant flu strain or mutates into a deadlier illness.

Sanofi-Aventis SA of Paris, Baxter International Inc. of Deerfield, Illinois, and GlaxoSmithKline Plc of London, are talking with world health authorities about producing shots, the agency said. Mexico's Cordova said the WHO is sending a specialist to Mexico today to begin developing a vaccine.

Authorities advised hand washing, hygiene and staying home if sick as the most effective ways to control the outbreak. The WHO and CDC said closing borders or killing animals are costly steps that wouldn't slow the spread of flu.