HONG KONG - The new H1N1 flu virus is expected to surge in coming months in the southern hemisphere when the winter season begins, health experts said, calling for continued vigilance even if the virus appears to be mild.

Influenza viruses thrive in colder months. Winter is also a time when flu infections surge because people tend to crowd around in poorly ventilated settings.

Yes, it is quite possible that more cases will be seen in the southern hemisphere as winter comes, that's really just starting now, said Bill Rawlinson, head of virology at the Prince of Wales Hospital in Sydney, Australia.

It does appear mild but every year in Australia, we have 1,500 to 3,000 deaths due to flu. When you get a new strain, even if it may not be as fatal as H5N1, it is still a serious event and you do see an excess of deaths due to new strains because people don't have immunity, he told Reuters by telephone.

The new virus, which broke out in Mexico, has now infected more than 1,200 people in 21 countries. To date, 27 deaths have been confirmed, 26 in Mexico and one in the United States.

While many scientists agree this new virus does not appear more deadly than seasonal human flu, WHO chief Margaret Chan called for caution on Monday, saying it was still early days and no one could tell how the situation or the virus would evolve and if the world was headed for a pandemic.

CRITICAL PERIOD

Raina MacIntyre, professor of infectious diseases epidemiology at the University of New South Wales, says a worst-case scenario would be an epidemic hitting countries such as Australia and New Zealand.

The worst case scenario is an epidemic that could cause illness in 40 percent of the population, cause 50 percent work and school absenteeism, and 2.4 percent case fatality rate. With our best available strategies for prevention and mitigation, we could reduce that to 10 percent getting sick and halve the case fatality rate, she told Reuters in an email.

Yes, we are definitely more at risk given we are coming into our winter, when influenza transmission is intensified ... we are in a critical phase right now.

But no one has a clue how the virus will behave from here.

So far, we have not seen any particular virulence with this virus, though more cases need confirmation to be sure of this. This also means that it can be a candidate for reassortment with other potentially more virulent strains of influenza, such as avian H5N1, said Julian Tang, consultant with the Division of Microbiology at National University Hospital in Singapore.

Some experts consider that to be the worst that can happen. The world would be faced with a monstrous virus that not only spreads with ease among people, but packed with the killing power of the H5N1, which has a mortality rate of 60 to 70 percent.

TROPICS: BEWARE AIR CONDITIONING

Tang said in the tropical countries, modern comforts, like air-conditioning, may fuel the spread of H1N1.

With tropical countries using air-conditioning more heavily in hot weather ... outdoor climate factors may become irrelevant when most of influenza (and other respiratory viruses) are transmitted indoors in an air-conditioned environment in tropical countries, he said.

There is no seasonal pattern to influenza in the tropics and outbreaks happen all year round.

Purely in terms of airborne influenza virus survival, opening the windows so that the hot, humid air comes in, will reduce virus survival - but it will be damn uncomfortable, said a microbiologist in Asia, who declined to be identified

For now, public health experts are mostly recommending rigorous disease prevention measures but the most powerful weapon may be past exposure to seasonal human H1N1.

Past infections (particularly with human H1N1 influenza) may have induced some antibodies that may cross-protect against some other common antigens (e.g. internal viral proteins) that are more similar to those in the new H1N1 (2009) virus, Tang said.

Australia Mexico Medicine