Sao Paulo, Brazil ? The Southern Hemisphere has been mostly spared in the swine flu epidemic. That could change when winter starts in coming weeks with no vaccine in place, leaving half the planet out in the cold.

So far, the most affected nations have been in North America and Europe, which are heading into summer. But flu is spread more easily in the winter, and it’s already fall down south. Experts fear public health systems could be overwhelmed — especially if swine flu and regular flu collide in major urban populations.

“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.

There’s also a chance that the two flus could collide and mutate into a new strain that is more contagious and dangerous.

“We have a concern there might be some sort of reassortment and that’s something we’ll be paying special attention to,” World Health Organization spokesman Dick Thompson said.

Flu spreads more readily during the winter because people congregate indoors as the weather gets colder, increasing the opportunity for the virus to hop from person to person, said Raina MacIntyre, public health director at the University of New South Wales in Australia. Colder temperatures also may make it easier for the virus to infect people.

“The highest peaks of influenza activity occur in winter,” MacIntyre said. “For us in the Southern Hemisphere, it’s particularly concerning.”

And while New Zealand is the only southern nation with confirmed swine flu cases, “it’s almost inevitable that it will come to Australia,” she said. Health officials in Brazil also say it’s a near-certainty swine flu will hit Latin America’s largest nation, where there are 25 suspected cases but none confirmed so far.

Humans have only limited natural immunity to this new blend of bird, pig and human viruses, but the strain has killed relatively few people in its current form compared with traditional flu, which kills about 36,000 people each year in the U.S. and more than 250,000 worldwide.

The timing is particularly challenging for vaccine makers. A vaccine for swine flu is still months from being produced, and will likely be available just as flu season is ending in southern countries.

“The vaccine won’t come in time for South America,” said Dr. Gonzalo Vecina of Sao Paulo’s prominent Hospital Sirio-Libanes.

In addition, many companies may switch to making swine flu vaccine instead of seasonal flu vaccine, jeopardizing the southern countries’ regular flu vaccine stocks. That could mean fewer seasonal flu vaccines available for next year’s Southern Hemisphere winter.

“This is a concern we are working on,” Andrus said. “We want to prevent it from being a potential barrier to getting it to the people who need it most.”

Even in normal years, vaccine makers don’t have the capacity to make enough courses for more than a fraction of the world’s population.

Some experts think health officials in Southern Hemisphere countries should be more concerned with seasonal flu than with swine flu.