The new head of the World Health Organization took office Thursday, urging the world to remain vigilant against the threat of H5N1 avian influenza and warning that complacency endangers efforts to prepare for the next influenza pandemic.

Dr. Margaret Chan also said WHO will review its polio eradication strategy with scientists, financial donors and affected countries in coming months to try to determine how to finally complete the 18-year-old campaign to drive the polio virus out of humankind worldwide.

"We need to have a balanced view," Chan said of the threat that H5N1 might trigger a flu pandemic.

"One should not take the alarmist approach. And on the other hand, complacency is our biggest enemy."

Chan, 59, holds Chinese and Canadian citizenship, and received her medical degree from the University of Western Ontario. She was nominated for the WHO's top job by China and is the first Chinese citizen to hold such a high-ranking UN position.

She comes to the director generalship after serving as head of the WHO's pandemic preparations.

With a slowing of the rate of new human infections in the past few months, the sense of pandemic alert has subsided in many quarters. But Chan said the recent relative calm on the H5N1 front should be kept in perspective.

"My sense is that, yes, in the past six months perhaps there is diminished sense of the importance of H5N1. But this sense is being awakened again given the recent events in Vietnam, in also Korea and so on."

Those two countries are currently battling outbreaks of the virus in poultry. In South Korea's case, the outbreaks are the first since 2004. Vietnam, one of the countries that has been hardest hit by H5N1, had gone more than a year without outbreaks in birds and without human cases, only to see the virus flare up again in poultry late last year.

And just before Christmas, three people in Egypt, all members of an extended family, died after contracting the virus. According to the most recent WHO tallies, 261 people have been infected since late 2003 and 157 of them have died.

"We learned from past experience it goes into cycles. There would be periods of high H5N1 activity or low activity," Chan said from Geneva during a telephone news conference.

"These kinds of what I call peaks and troughs are very characteristic for influenza. ... So my advice to health authorities would be: We are moving into the peak season of influenza activity again. This is the time we really need to redouble our efforts in terms of heightened sense of vigilance."

Chan also acknowledged it is time to review efforts to eradicate polio, suggesting more international funding and increased commitment from the governments of affected countries are needed.

"Yes, I'm concerned with the need to really take a re-look at eradication of polio," she said. "We need to ask ourselves the honest questions. Do we have the tools to do it?"

Huge strides were made in the years leading up to the original target date for eradication, 2000. But in the past few years, case counts have climbed. Last year 1,874 cases of paralytic polio were reported, compared to 1,749 in 2005. In 2001, there were only 483 cases.

The new director general suggested that new oral vaccines aimed at individual subtypes of polio are proving more effective than the vaccine they replaced, which targets all three strains at once. And she said political will is improving "but we need more commitment from the countries affected."

"Do we have the right kind of resources to do the job?" Chan said the answer to that question is "No."

The program currently faces a US$450 million shortfall for the 2007-2008 period. The polio eradication initiative, a co-operative effort between WHO, Rotary International, the U.S. Centers for Disease Control and UNICEF, was originally expected to be completed for a cost of US$4 billion. Officials suggest the total bill will be closer to US$5 billion.

"We must have a very honest discussion with the academic community, the scientific community, the donors and also the countries involved: what kind of actions we need to put in place in the next 24 months in order to achieve the target," Chan said.