WASHINGTON - The United States will face an H1N1 flu vaccine shortfall of 45 to 55 million doses by the end of the year but half of the population could still be vaccinated, a senior US official said Monday.

"I don't think we will get to the original goal" of 195 million influenza A(H1N1) vaccine doses delivered during the US government's fall vaccination campaign, said Anthony Fauci, Director of the National Institute of Allergy and Infectious Diseases.

But he tempered that the government may obtain 140 to 150 million doses, "which quite frankly I think will likely be enough because we don't anticipate more than half of the people want to get vaccinated.

"If we get to 150 million, we will likely have as much as anybody needs," Fauci told AFP.

As of Friday, 16.1 million doses of H1N1 flu vaccine were ready for shipping, and over 11 million doses had been sent out to state health authorities.

Health Secretary Kathleen Sebelius has already warned that demand is outstripping supply of vaccine for the novel flu strain, as 46 of the 50 states now report widespread A(H1N1) activity at an unusually early time of the year.

"What we have seen in the US is a significant dichotomy or gap between the demand and the supply," warned Fauci. President Barack Obama has declared a national emergency for doctors and nurses to better deal with the rapid spread of the virus that has infected millions and killed over 1,000 people in the country.

But Fauci insisted the vaccine shortage would not change Washington's promise to donate 10 percent of its doses to the World Health Organization for distribution to the poorest countries.

"That pledge is still valid but we have to think to make sure that we get the most vulnerable in our population taken care of before the release of 10 percent of our vaccines," he said.

Fauci explained that manufacturers are facing difficulties delivering as much vaccine as expected because of the lengthy amount of time needed to grow the virus in chicken eggs for vaccine compared to seasonal flu strains.

"Unfortunately for everyone - not only in the US but around the world - this is a very slow-growing virus," he added.

"Once it grows, it makes a very good vaccine that is quite effective and safe. But the projected dosage we thought that we would have by this time of the fall is far less that we had anticipated."

At least 4,999 people have died from H1N1 flu infections worldwide since April, when an outbreak was first reported in Mexico before rapidly spreading to the United States, according to the World Health Organization.