An electron microscope image shows an A H1N1 swine flu virus culture obtained from a California patient in April 2009. (C. S. Goldsmith and A. Balish/Centers for Disease Control/Reuters)

Seasonal flu is killing more people in Argentina than swine flu, say health officials who are closely watching how the virus behaves during the Southern Hemisphere's winter.

Argentina has a month left in its winter flu season. Swine flu infections seem to have peaked, as trips to doctors' offices by people with flu symptoms have declined.

The approaching end of regular flu season in the Southern Hemisphere could help in tracking the spread of swine flu in the region, WHO spokesperson Thomas Abraham said Friday.

Symptoms of swine flu and seasonal flu are very similar and laboratories are unable to test all cases.

Once the regular flu season ends, officials will be able to assume all flu cases are swine flu or H1N1, as in the Northern Hemisphere, he said.

Likely greater impact on young

Genetic sequencing from two Argentine samples tested so far show the H1N1 pandemic virus is virtually the same as what is circulating in the U.S. and Mexico. Scientists are checking for any signs the virus could be mutating into a more dangerous form.

As in other countries, young, healthy people in Argentina are getting sicker and dying more often from swine flu than are other age groups.

"Compared to other influenzas, the death toll won't be any greater, but we are probably going to see a much greater impact on the young," said Dr. Vilma Savy, who runs the respiratory virus service at Argentina's National Institute of Infectious Diseases.

Argentina has the most swine flu deaths outside of the U.S. Like other countries, it has stopped publishing death tolls.

Watch for danger signs

In a pandemic influenza briefing on Friday, the World Health Organization noted U.S. research published in The Lancet highlights an increased risk of severe or fatal illness in pregnant women infected with the H1N1 pandemic virus.

The agency also recommended starting antiviral treatment as soon as possible after symptoms begin.

Worldwide, most patients infected with the pandemic virus continue to experience mild symptoms and recover fully within a week, even without medical treatment.

"Within this largely reassuring picture, a small number of otherwise healthy people, usually under the age of 50 years, experience very rapid progression to severe and often fatal illness, characterized by severe pneumonia that destroys the lung tissue, and the failure of multiple organs," the briefing note said.

"No factors that can predict this pattern of severe disease have yet been identified, though studies are underway."

Besides pregnant women, people with underlying medical conditions, such as chronic lung disease (including asthma), cardiovascular disease, diabetes and weakened immune systems are also at increased risk of severe or fatal illness.

The agency said clinicians, patients and home-based care providers should watch for danger signs of progression to more severe disease:

• Shortness of breath, either during physical activity or while resting.
• Difficulty in breathing.
• Turning blue.
• Bloody or coloured sputum.
• Chest pain.
• Altered mental status.
• High fever that persists beyond three days.
• Low blood pressure.

In children, danger signs include fast or difficult breathing, lack of alertness, difficulty in waking up and little or no desire to play.

With files from The Associated Press and The Canadian Press