SAO PAULO (AP) - 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 in Geneva.
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 the never-before-seen H1N1 swine flu virus, which is a blend of bird, pig and human viruses that jumped from pigs to humans and began spreading easily. The strain has killed relatively few people in its current form compared to 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, potentially jeopardizing the southern countries' regular flu vaccine stocks for next year.
"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 shots 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.
John Mackenzie, a flu expert at Curtin University in Australia, said countries should focus on regular flu vaccines for high-risk populations, including the elderly and those with chronic illnesses, since swine flu appears relatively mild so far.
But Thompson said WHO is also concerned about a possible "reassortment" - or mixing of regular and swine flu viruses.
"Governments have to step up their actions to protect their populations, especially in the absence of a (swine flu) vaccine," said Thompson. "Latin American countries may have a somewhat stronger surveillance system than in Africa. Africa's going to need some additional support and surveillance."
Brazil on Monday announced it was authorizing $67 million in emergency funding to combat swine flu, much of it for public information campaigns on how people can prevent its spread through basic means, like frequent hand-washing.
In Africa, which has yet to confirm a swine flu case, an outbreak during traditional flu season will make diagnosing and treating the two viruses a challenge, said Barry Schoub, director of South Africa's National Institute for Communicable Diseases.
Even in the absence of cases, officials are preparing. Johannesburg's O.R. Tambo International Airport, a regional gateway that handles millions of travelers each year, has plans to get a thermal image detection system running to check passengers for fever. A supply of masks has been provided to that airport and others, as well.
Hospitals have been given guidelines on how to handle suspected cases. South Africa, the richest country in the region, is poised to assist its neighbors should they need help with testing or treatment.
South Africa has stockpiled about 100,000 courses of the antiviral drug Tamiflu, used to treat those infected, and has access to more if needed, Schoub said.
Other countries said they're well-prepared, too. Australia has a stockpile of 8.7 million courses of Tamiflu and Relenza to treat its population of 22 million, MacIntyre said. Brazil says it is well-prepared but has Tamiflu for just 9 million people in a nation of more than 190 million.
Argentina, population 40 million, has 500,000 treatments with another 110,000 on order. Chile, with 16 million, has 300,000 treatments and has asked for 500,000 more. Venezuela has boasted of having plenty of Tamiflu but has not responded to repeated requests to say how much is available for the nation of 26 million.
And in Bolivia, one of the hemisphere's poorest nations, Health Minister Ramiro Tapia announced Monday night that the country has only 100 treatments but that WHO has promised an emergency shipment of 12,000. Tourists feared they might not be given Tamiflu if they fall ill, but Tapia said the government would provide it free to anyone in need.
The greatest risk to South American nations are its most vulnerable populations, who live in slums ringing big cities and often have little access to health care.
"You can't talk about at-risk countries, but rather populations at risk, and that's the families of eight people who live together in a single room," said Dr. Mauricio Espinel, an epidemiologist at Ecuador's University of San Francisco.
Contributing to this report: Associated Press writers Kristen Gelineau in Sydney; Maria Cheng in London; Donna Bryson in Johannesburg; Gonzalo Solano in Quito, Ecuador; and Carlos Valdez in La Paz, Bolivia.