May 5 The virulence of the swine flu that has sickened at least 1,085 people in 21 countries may reveal itself when the Southern Hemisphere faces its influenza season that begins this month, global health authorities say.

The World Health Organization said it's bracing for the possibility the disease will worsen despite reports that symptoms are no more severe than seasonal flu. The outbreak is easing in Mexico, where it struck hardest, Health Minister Jose Cordova said yesterday. That shouldn't lead world governments to lessen their efforts to distribute medicines and develop a swine flu vaccine, the WHO said.

The onset of warmer temperatures may be helping quell the flu spread in Mexico, said Miguel Angel Lezana, director of the National Center for Epidemiology Control in Mexico City. Scientists will watch outbreaks in the Southern Hemisphere to see if swine flu, formally known as influenza A (H1N1), becomes the dominant strain and how much the virus will mutate.

"People who tell you what flu is going to do have no idea what they're talking about," Michael Osterholm, a flu expert at the University of Minnesota in Minneapolis, said yesterday in a telephone interview. "We have no idea."

While small outbreaks may occur in schools or other closed settings, community-wide spread of flu has never been seen in the Northern Hemisphere from May through August, said Ira Longini, a University of Washington statistician who advises the U.S. government on influenza spread. The burst of cases in Mexico in April may be winding down with the end of flu season, he said.

‘Encouraging Signs'

"While we're not out of the woods yet, we are seeing some encouraging signs," said Richard Besser, acting chief of the U.S. Centers for Disease Control and Prevention in Atlanta, in a conference call yesterday. "Here in the United States, we're at the very end of our flu season. There aren't a lot of other flu viruses around that it's competing with. What happens when it goes into countries where there are other viruses circulating?"

Global health authorities will monitor those countries in the Southern Hemisphere during the flu season that stretches through September, said Keiji Fukuda, the World Health Organization's assistant director-general for health, security and environment.

"We're really trying to get a handle on how far has this virus spread out and how much has it established itself in different parts of the world," Fukuda said yesterday at a news conference in Geneva.

Similar Rate

In Mexico, swine flu is spreading at about the same rate as seasonal flu, with each new patient infecting, on average, about 1.5 people, said Lezana. The number is an early estimate and may change, he said.

Swine flu is suspected in 44 of the 50 U.S. states, and the infected may number in the thousands, U.S. health officials said. Even if symptoms are mild, the ease with which the new virus spreads among a world population with no natural immunity makes it a threat, the CDC said.

"We're cautiously optimistic this particular strain will not be more severe than a normal seasonal flu outbreak," Janet Napolitano, the U.S. secretary for homeland security, told reporters in Washington yesterday. The "large majority" of the cases are "mild," have not required hospitalization, and many of the patients have recovered, she said.

Affected Countries

The WHO, a Geneva-based agency of the United Nations, yesterday added Colombia and El Salvador to the list of countries with confirmed cases. The other nations are Austria, Canada, Costa Rica, Denmark, France, Germany, China (Hong Kong), Ireland, Israel, Italy, Mexico, the Netherlands, New Zealand, Portugal, South Korea, Spain, Switzerland, the U.K., and the U.S.

In the U.S., H1N1 has been confirmed by laboratory tests in 286 patients in each of nine regions tracked by the U.S. census, the CDC reported. There are 101 cases in Canada, according to the WHO. Mexico said late yesterday it had confirmed 26 deaths among its 802 cases.

Data so far suggest the virus is striking younger patients than is typical for influenza, and younger patients than usual are entering hospitals, said Anne Schuchat, a scientist at the CDC. "Very few" patients with swine flu are older than 50, and the median age is 17. It's possible that older people have greater immunity, she said in a conference call on May 3.

School Strategy

The U.S. strategy to close schools with suspected cases doesn't effectively halt the virus' spread once it becomes established in an area, Besser, of the CDC, said. The agency is evaluating whether to change its advice to schools, he said.

About 533 schools in 24 states in the U.S. were closed yesterday for the flu, shutting out about 330,000 students, according to the U.S. Department of Education.

While there is no vaccine against swine flu, shots are the most efficient and cheapest way to fight a pandemic, said Longini, the biostatistician who advises the government on flu. Closing schools, for example costs about 20 times more, mainly because of lost work hours and services, he 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. The CDC says people with the swine flu are more likely to have diarrhea.

The U.S. is hastening production of its annual flu shots based on strains identified before the H1N1 outbreak, said Kathleen Sebelius, who was confirmed as the U.S. Health and Human Services secretary last week. That will make capacity available if vaccines are needed for swine flu, she said.

Batches of seed virus are being developed for potential vaccine production, according to the WHO. 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.

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.