(AP) -- Southern hemisphere countries that have largely escaped swine flu infections could soon become more vulnerable, experts warn, as the approaching winter brings with it an elevated risk of the virus spreading and mutating.
So far, the worst affected nations - such as Mexico, the United States, Canada and countries in Europe - have been in the northern hemisphere, which is heading into summer. To the south of the equator, cooler weather is imminent.
"The highest peaks of influenza activity occur in winter," said Raina MacIntyre, head of the University of New South Wales' School of Public Health and Community Medicine. "For us in the southern hemisphere, it's particularly concerning."
It is already fall in Australia, and winter sets in by June.
Flu is spread more easily in winter largely because people tend to congregate indoors to avoid the colder weather, increasing the opportunity for the virus to hop from person to person, MacIntyre said. There is also some evidence that colder temperatures make it easier for the virus to infect people, she said.
Experts also warn that regular flu that typically spreads in winter may collide with swine flu and recombine to make it more transmissible or more dangerous.
WHO spokesman Dick Thompson, said the agency was concerned of a possible "reassortment" - or mixing of regular and swine flu viruses.
"Winter is coming in the southern hemisphere and governments have to step up their actions to protect their populations, especially in the absence of a (swine flu) vaccine," he said. "We have a concern there might be some sort of reassortment and that's something we'll be paying special attention to."
So far, Australia has reported no confirmed cases of swine flu.
A WHO spokesman said Monday the agency may raise its pandemic alert to the highest level - 6 - meaning a global outbreak of swine flu is under way.
However, U.N. Secretary-General Ban Ki-moon said the World Health Organization "has no plan to raise the alert level to 6 at this moment." WHO chief Margaret Chan also told the U.N. General Assembly by videolink from Geneva that "we are not there yet."
If WHO does declare a pandemic, "it's almost inevitable that it will come to Australia," said MacIntyre. "If the WHO does not declare Level 6 there's a possibility the virus will never reach Australian shores."
The virus has already reached neighboring New Zealand, which reported its fifth and sixth laboratory-confirmed cases of swine flu Monday along with 11 probable cases. The two new confirmed cases came 10 days after a group of high school students returned from Mexico with the first confirmed cases of the illness, sparking a nationwide alert.
Mexico has so far reported 727 cases of swine flu - out of more than 1,000 confirmed worldwide - and 26 deaths from the virus.
But even as Mexico began its first tentative steps toward normalcy after days of lockdown, the virus spread to Colombia in the first confirmed case in South America, where flu season was also about to begin.
"Latin American countries may have a somewhat stronger surveillance system than in Africa. Africa's going to need some additional support and surveillance," WHO's Thompson said.
Experts also warn the timing of the outbreak may jeopardize southern countries' regular flu vaccine stocks for next year. Usually, flu vaccines are made about a year in advance.
But since WHO will ask vaccine manufacturers in a few weeks to start making pandemic vaccine instead, that could mean problems for next year's stocks of seasonal flu vaccine for southern hemisphere countries. Vaccine makers can only make one kind of vaccine at a time, and many companies may switch to making swine flu vaccine instead of seasonal flu vaccine for southern hemisphere countries.
Some experts think health officials in southern hemisphere countries should be more concerned with seasonal flu than with swine flu at the moment. "Seasonal flu is ... potentially more serious than (swine flu)," said John Mackenzie, a flu expert at Curtin University in Australia.
He said countries like Australia should focus on having people at high risk of the flu, like the elderly and those with chronic illnesses, vaccinated against regular flu since the swine flu appeared to be relatively mild.
WHO is reporting no confirmed cases in Africa, but the region is bracing itself.
O.R. Tambo International Airport in Johannesburg, a gateway to the region handling millions of travelers, 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, and hospitals have been given guidelines on how to handle suspected cases.
South Africa, the richest country in the region, is also poised to assist its neighbors should they need help with testing or treatment.
"There's certainly an enhanced preparedness," said Barry Schoub, director of South Africa's National Institute for Communicable Diseases.
Schoub said South Africa has about 100,000 Tamiflu doses stockpiled and access to more if needed. However, he said, if the country experiences a swine flu outbreak at the same time as the coming flu season, it will put pressure on both treatment and diagnosis.
Australia is well-prepared for an outbreak, MacIntyre said. It has spent years planning for a pandemic, and has a large stockpile of flu treatments and solid pandemic preparedness plans, she said.
The government says its stockpile is large enough to treat 8.7 million of its 22 million people with the flu medicines Tamiflu and Relenza.
So far, these prescription drugs have been given to about 100 people identified as at-risk of swine flu because they are have flu-like symptoms and recently traveled to high-risk areas.
Endocrinologist Nikolai Petrovsky, a professor of medicine at Flinders University in Adelaide, expressed worries about the coming winter, but said that at least Australia will have more time to analyze the data coming out of the U.S. and Mexico before the virus arrives on its shores.
"By the time it comes to Australia and the southern hemisphere, we'll know more about it than it did when it arrived over there," he said.
Associated Press writer Maria Cheng in London and Donna Bryson in Johannesburg contributed to this report.
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Explore further: Doctors urge meningitis shots for vulnerable infants, children