Women wearing masks to protect against the transmission of swine flu ride a bus in downtown Mexico City on Tuesday. (Brennan Linsley/Associated Press)

State officials in Texas confirmed the first death of a U.S. resident from swine flu Tuesday, as the worldwide number of confirmed cases approached 1,500.

The victim was a woman who lived in Cameron County, on the border with Mexico. The woman, who hasn't been identified, had other chronic health problems. She died earlier this week, said officials with the Texas Department of State Health.

Last week, a boy from Mexico City visiting family in Texas died at a Houston hospital, marking the first swine flu death in the U.S.

Around the world, the number of confirmed swine flu cases stood at 1,490, with 30 deaths related to the outbreak of the new H1N1 virus, Keiji Fukuda, assistant director-general of the World Health Organization, said Tuesday.

Alberta girl 'recovering well'

In Canada, 25 more cases of swine flu were reported Tuesday, bringing the country's total to 165.

The new cases include:

• 10 new cases in Nova Scotia, bringing the provincial total to 48.
• 7 in British Columbia, for a total of 46.
• 5 new cases in Ontario, for a total of 36.
• 2 new cases in Alberta, for a total of 26.
• 1 new case in Quebec, for a total of 4.

All but one of the cases in Canada are mild, health officials said.

The one severe case involves a young Alberta girl who is being treated in an Edmonton hospital. Dr. André Corriveau, Alberta's chief medical health officer, said the girl is getting better and is breathing on her own.

"I can tell you she is recovering well," Corriveau said on Tuesday. "Her condition is improving."

He said he couldn't go into details about why the girl suffered serious symptoms when other cases in the country have been mild, although he noted that some underlying conditions can exacerbate influenza's impact.

"The prime reason [she was in the hospital] was the swine flu," he said. "But the other thing we're saying is there may be other factors that were part of the overall picture."

WHO discusses evolution of the virus

The WHO's scientific committee met virtually on Tuesday to discuss the evolution of the virus. Fukuda said health experts are on the lookout for cases of the virus transmitting in Europe beyond institutions such as schools or travel groups.

Workers dressed in protective suits disinfect a subway station during a demonstration for the media in Mexico City on Tuesday. (Eliana Aponte/Reuters)

"Right now, we don't believe we are seeing community transmission in the same way as we are seeing in the United States and Mexico," Fukuda said.

He also said the scientific team was examining whether older people had some level of protection from the virus, as younger people made up a large number of those infected so far.

But Fukuda also noted younger people tend to travel more, which could explain the higher rate of infection.

Mexico holiday celebrations cancelled

As Mexico prepared to return to normalcy, world health officials warned the global spread of swine flu is still in its early stages and countries should remain vigilant. On Tuesday, there were 840 confirmed cases in Mexico.

Mexico's Cinco de Mayo celebrations were cancelled on Tuesday as a five-day shutdown concluded.

Businesses were expected to begin reopening their doors on Wednesday. Universities and high schools were scheduled to follow on Thursday. Elementary schools won't reopen until May 11.

But experts inside Mexico's swine flu crisis centre warned that the virus remained active throughout Mexico and could bounce back as people return to work and school in the country that is considered the epicentre of the outbreak.

"It's clear that it's just about everywhere in Mexico," Marc-Alain Widdowson, a medical epidemiologist from the U.S. Centers for Disease Control and Prevention, told The Associated Press.

U.S. Secretary of Health and Human Services Kathleen Sebelius, left, cleans her hands with sanitizer as she tours the emergency operations centre of the Centers for Disease Control facility with acting CDC director Richard Besser in Atlanta on Tuesday. (John Amis/Pool/Associated Press)

Scientists said the virus is still spreading in the U.S. and that chances are the number of severe cases will rise there.

There are currently 403 confirmed cases of the virus in 38 states, the CDC said Tuesday.

"We are by no means out of the woods," said Dr. Richard Besser, acting director of the CDC.

Cases have been confirmed in 21 countries. In addition to Canada, the U.S. and Mexico, confirmations have been made in Spain, Britain, Germany, New Zealand, Italy, Israel, France, El Salvador, South Korea, Austria, Costa Rica, Colombia, Denmark, Hong Kong, Ireland, the Netherlands, Portugal and Switzerland.

Experts said the known cases were almost certainly only a fraction of the real total.

Concerns mount for Southern Hemisphere

The Southern Hemisphere may be particularly at risk as the region enters winter and its flu season, according to some medical experts.

Experts worry that seasonal flu viruses could combine with swine flu, creating a new strain that is more contagious or dangerous.

"You have this risk of an additional virus that could essentially cause two outbreaks at once," Dr. Jon Andrus said at the Pan American Health Organization's headquarters in Washington.

But the WHO has urged governments to avoid unproven actions to contain the disease, including group quarantines of travellers from Mexico and bans on pork imports.

China, Argentina and Cuba are among the nations banning regular flights to and from Mexico.

China has also banned the import of pork from Mexico, some U.S. states and Alberta, despite the WHO assuring that the virus is not spread through the consumption of pork.

Several North American travellers have also been put under preventive quarantine in China.

The WHO is still studying whether to raise the global pandemic alert to its highest level.

The global pandemic alert is at Phase 5, its second-highest level. Phase 5 is called when there is human-to-human spread of a virus in at least two countries in one region, according to WHO's pandemic response guidelines.

The classification means a pandemic is imminent and countries must finalize preparations to deal with the outbreak of swine flu.

To declare a full-blown pandemic, WHO would have to be convinced the new virus is spreading in a sustained way among communities in another region besides North America.

"We do not know how long we will have until we move to Phase 6," WHO director general Margaret Chan said. "We are not there yet. The criteria will be met when we see in another region outside North America … very clear evidence of community-level transmission."

Health officials have said that a Phase 6 alert is not a reason for panic.

A pandemic can refer only to geographic spread and not the severity of the illness. The two most recent pandemics, in 1957 and 1968, were relatively mild.

With files from The Associated Press