*to access the full regional surveillance of New Zealand, please visit http://www.esr.cri.nz/frame.html?content=/features/surveillance/flu/fluindex.html

As Americans scramble for scarce doses of flu vaccine in hopes of warding off the respiratory bug this winter, the wily influenza virus may have other plans.

Through a natural process known as antigenic drift, a new strain of influenza that can diminish the effectiveness of today´s vaccine is already emerging on the far side of the world.

A shortage of vaccine occurred when British regulators found contamination problems at a Chiron Corp. factory in Liverpool, effectively wiping out about half the U.S. supply. The shortage has triggered a run on the remaining doses and a struggle by health authorities to keep the panic in check.

Officials said last week they are optimistic that those who do manage to get a flu shot will be protected. But predictions are impossible, given the way the virus manages to dodge its attackers.

The emergent strain raising questions now is known as A/Wellington, named after the New Zealand city where it was first detected.

"The flu season has been late this year, and it seems some people who have been vaccinated have been hit by this changing strain,创 said New Zealand Health Ministry flu chief Dr. Lance Jennings.

Like weather forecasters watching tropical storms, epidemiologists for the World Health Organization track the ever-evolving strains of influenza. The predominant flu virus around the globe right now is one called A/Fujian, and the vaccine Americans are seeking today is a perfect match for it.

But A/Wellington is gaining ground. Tests suggest that 43 percent of recent New Zealand flu cases spring from the new strain, or variants of it.

A/Wellington has even turned up about as far from the South Pacific as is geographically possible: in Norway.

So convincing was the late season surge of A/Wellington that the WHO on Oct. 8 recommended that next year´s flu vaccine for the Southern Hemisphere, which is shipped in March, be reformulated to protect against it.

Dr. Nancy Cox, director of the Influenza Branch at the Centers for Disease Control and Prevention, said there is no way of knowing whether the A/Wellington strain will establish a beachhead in the United States this winter.

"Influenza viruses are inherently unpredictable,创 she said in a telephone interview.

One reason for concern is that laboratory animal tests suggest that the current vaccine -- which targets A/Fujian -- is about two-thirds less effective in stirring antibodies against A/Wellington than it is against the targeted strain.

That does not mean, Cox emphasized, that the current flu shot would be two-thirds less effective in actual use, should the A/Wellington strain turn up in the United States. People who regularly receive flu shots, she said, may have higher levels of antibody protection against influenza than the laboratory animals.

"We would not anticipate that the Wellington strain will cause an enormous problem,创 she said.

Doris Bucher, a flu vaccine expert at New York Medical College in Valhalla, N.Y., said the animal tests on A/Wellington are actually encouraging because they show that antibodies stirred up by today´s vaccine will inhibit the new strain.

"If Wellington rushed in and took over, people would have good protection with the current vaccine,创 she said.

Bucher´s lab specializes in developing strains of influenza that grow well in eggs and hence serve as the basis for each year´s flu vaccine made from eggs. The current flu vaccine is made from a strain developed by her laboratory, and she is working right now on an A/Wellington-like strain for manufacturers who will make next year´s vaccines.

When flu vaccines are well-matched to the prevailing flu strains, the shots can prevent flu in 70 percent to 90 percent of vaccinated adults, according to the CDC. Well-matched shots may prevent flu in only 30 percent to 40 percent of nursing home residents, but they can reduce the death rate from influenza and pneumonia in that population by 80 percent.

Last year, the vaccine was a less than perfect match.

American manufacturers need a nine-month lead to produce flu vaccine. That requires flu forecasters to predict in February what the prevailing flu strain will be November.

In February 2003, the Food and Drug Administration´s Vaccines and Related Biological Products Advisory Committee selected a strain called A/Panama as the target. Another strain, known as A/Fujian, was starting to emerge on the World Health Organization´s flu radar, but planners feared there was not enough time to develop a vaccine for it.

Influenza struck the United States early and hard last year. The first cases turned up in early October in Texas, and soon anxious Americans were clamoring for vaccine -- a prelude to this year´s flu shot frenzy.

A near-record 83 million eventually had a flu shot last fall. But the predominant flu strain was not A/Panama -- it was A/Fujian. And the vaccine was only partially effective.

A Colorado study for the CDC concluded last year´s flu shots were 52 percent effective in protecting healthy adults against flu, 38 percent effective in preventing flu among those with health conditions putting them at higher risk.

Bucher said it is reassuring that tests are showing there is less of a mismatch between the current flu vaccine and A/Wellington than there was last year between A/Panama and A/Fujian.

"It´s nothing like the situation we had last year,创 she said.

The issue, of course, will be academic if the A/Fujian strain predominates again this year -- as it did in the Southern Hemisphere during most of the winter season that has now ended there. Those 61 million Americans who will find a vaccine will be protected.

There is also some evidence that the A/Fujian strain -- having swept through the globe last year -- may have lost its punch. Despite the late emergence of the new flu strain, influenza was unusually mild throughout the Southern Hemisphere from May through October.

One reason may be that the human population, vaccinated or not, developed its own natural resistance to A/Fujian when it came through last winter.

"There are a number of factors that determine whether you will have a good, medium or bad flu year, and clearly a major factor is whether the predominant strain happens to be the same as the one the previous year,创 said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

So far, there have been only isolated cases of flu detected in the United States at the start of this year´s flu season, and all have turned out to be A/Fujian.

"Things are looking pretty good, in that regard,创 Fauci said. "But that doesn磘 mean we won磘 get into trouble in November or December.

"It´s much too early,创 he said. "to predict what kind of year we are going to have.创