CHICAGO (Reuters) - The new H1N1 virus appears to outcompete seasonal flu, making it less likely to mix with other circulating flu viruses into a “superbug” as some had feared, U.S. researchers said on Tuesday.

A couple wearing protective masks rest as they wait for their flight at Kingsford Smith International Airport in Sydney June 17, 2009. REUTERS/Daniel Munoz

The H1N1, or swine flu, virus also spreads more quickly and causes more severe disease in animal studies, the team said, but it shows no signs of mixing with either of the two seasonal flu viruses to form a new, so-called reassortant virus.

“The results suggest that 2009 H1N1 influenza may outcompete seasonal flu virus strains and may be more communicable as well,” said Dr. Anthony Fauci, of the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health.

The study, conducted in ferrets by a team at the University of Maryland, underscores the need for getting vaccinated with the new swine flu vaccine, Fauci said in a statement.

When the team infected ferrets with 2009 H1N1 virus plus either seasonal H1N1 virus or seasonal H3N2 virus, both viruses made them sick, but only the H1N1 virus spread to other ferrets, suggesting it will dominate ordinary flu.

“The H1N1 pandemic virus has a clear biological advantage over the two main seasonal flu strains and all the makings of a virus fully adapted to humans,” Daniel Perez of the University of Maryland said in a statement.

“I’m not surprised to find that the pandemic virus is more infectious, simply because it’s new, so hosts haven’t had a chance to build immunity yet. Meanwhile, the older strains encounter resistance from hosts’ immunity to them,” Perez said.

The researchers also found that the pandemic virus established infections deeper in the ferrets’ respiratory system, including the lungs, while the seasonal flu strains remained in the nasal passages.

That confirmed observations in people.

“The findings of this study are preliminary, but the far greater communicability of the pandemic virus is a clearly blinking warning light,” he said.

HARDLY ANYONE IMMUNE

The World Health Organization predicts a third of the world’s population will eventually be infected with swine flu.

Seasonal flu infects between 5 percent and 20 percent of a given population every year, but 90 percent of severe cases and deaths are among the elderly. It kills between 250,000 and 500,000 people globally.

Perez and his team used samples of the H1N1 pandemic variety from last April’s initial swine flu outbreak.

They found some of the animals infected with both a seasonal flu strain and the pandemic strain developed both respiratory and intestinal illness. The team plans to study whether this combination may explain some of the deaths attributed to the new virus.

Because hardly anyone has immunity to the new H1N1 virus, experts believe it will infect far more people than usual, as much as a third of the population.

Five companies are making swine flu vaccine for the U.S. market -- AstraZeneca’s MedImmune unit, CSL Ltd, GlaxoSmithKline Plc, Novartis AG and Sanofi-Aventis SA. Tests have begun to determine if people will need one or two doses to be protected.

Despite calls by scientific advisers to President Barack Obama to speed up supply, swine flu vaccines are unlikely to be ready before October, the U.S. Centers for Disease Control and Prevention said last week.