ALLENTOWN — Pennsylvania public health officials say they have been preparing intensively for a flu pandemic that in a worst-case scenario could strike nearly 13 percent of the population and cause some 9,000 deaths statewide.

With the Bush administration acknowledging Wednesday that states and cities will largely be on their own in the event of a massive outbreak, state health officials said they have been fine-tuning their Influenza Pandemic Response Plan for months.

"Pennsylvania is in a better state of preparedness than we were a year ago," said Michael Huff, a Health Department official who is leading the state's flu planning effort. But he added: "We are clearly not where we want to be yet. Right now, it's going to be a challenge if it happens tomorrow."

Some experts worry that a flu pandemic could arise from an Asian bird flu strain called H5N1, which has killed 113 people worldwide since 2003. There have been no documented cases of H5N1 in the United States, but experts worry the virus could mutate into a form that is easily transmitted from human to human.

Regardless of how it starts, a flu pandemic could strike an estimated 1.6 million Pennsylvanians, with about 39,000 people requiring hospitalization and 9,000 dying over the course of 12 to 16 weeks, according to the state's pandemic response plan.

Pennsylvania's $800 million poultry industry is vulnerable to H5N1 and other forms of avian influenza. But industry officials say that lessons learned from previous outbreaks of bird flu will help them prepare for the next one.

Poultry producers in Pennsylvania conduct more than 240,000 surveillance tests a year, more than any other state. An average of about 10 to 15 cases of bird flu are discovered each year in Pennsylvania, but in strains not considered dangerous to humans.

A 1983 outbreak of avian influenza led to the destruction of 17 million birds in Pennsylvania, Maryland and New Jersey, hobbling the industry for two years. A 1997 outbreak infected 21 Lancaster County flocks composed of 2.6 million birds, and more than 1 million had to be destroyed.

Should bird flu mutate into a form that strikes Pennsylvania's human population, the state's pandemic response plan calls for as many as 588 centers where residents could get vaccinated or receive anti-viral drugs such as Tamiflu.