Avian flu poses the single biggest threat to the world right now and health officials may not yet have all the tools they need to fight it, the head of the Centers for Disease Control and Prevention said on Monday.

Vaccine efforts are still focused on garden-variety influenza, which kills 36,000 Americans every year, and it would be impossible, in case of an avian flu epidemic, to switch gears quickly to make a special avian flu vaccine, CDC Director Dr. Julie Gerberding said.

"This is a very ominous situation for the globe," Gerberding told a meeting of the American Association for the Advancement of Science, calling it the "most important threat that we are facing right now."

"I think we can all recognize a similar pattern probably occurred prior to 1918," she said, referring to the 1918 pandemic of influenza, which also passed from birds to people and killed between 20 million and 40 million people globally.

The H5N1 avian flu, which first appeared in Hong Kong in 1997 and has since popped up twice, is evolving and can jump directly from birds to people, killing an estimated 72 percent of diagnosed victims, Gerberding said. Officials have documented 45 deaths so far from avian flu.

Gerberding said influenza was far more infectious than severe acute respiratory syndrome or SARS, which swept out of China in 2003, killing 800 people and causing global concern before it was stopped.

Health experts have also pointed out influenza kills much faster than diseases such as AIDS, taking tens of millions of lives in the space of weeks or months.

The "high season" for the avian flu was just starting in Asia, Gerberding said.

"We have this highly pathogenic strain circulating widely in poultry and ducks. There are really wonderful opportunities for this virus to either reassort (mix) with human strains of influenza or with other avian species," she said.

Hong Kong authorities stopped H5N1 in 1997 by sacrificing much of the poultry population in the territory. That was harder to do now that it had spread to Thailand, Vietnam, Cambodia and other countries, Gerberding said.

"People depend on poultry for their livelihoods and for feeding their families," she said.

The U.S. government contracted with two companies that already make flu vaccine -- Chiron Corp. and Sanofi Aventis to make an avian flu vaccine, which will begin testing in people later this year.

But this year´s influenza vaccine shortage caused by contamination problems at Chiron´s British plant showed how tenuous vaccine-making capability was, Gerberding said. Only three companies make influenza vaccine for the U.S. market.

"There is no wiggle room here," she said. Making an avian flu vaccine in case of an outbreak would be faster than starting from scratch, she said. "But we just don´t have the surge capacity to produce both."

So avian flu vaccine would be rationed.

"The very first doses would target the place where the outbreak is occurring," Gerberding told reporters. Health officials would use a "ring vaccination" strategy similar to that used to eradicate smallpox in the 1960s and 1970s, where exposed people and those around them are vaccinated.

People transmit flu before they become ill themselves so it would be almost impossible to stop it by watching or isolating sick people, Gerberding said.

Health experts are working to learn as much as they can about avian flu, such as doing blood surveys of healthy people in avian flu-affected areas.

"Are there people who have been exposed to the avian virus who have not gotten ill so we know what the true denominator is?" she asked.