A British drug manufacturer announced Wednesday that it has developed what appears to be the most effective bird flu vaccine created so far, and that it can be made in sufficient quantities for widespread distribution in case of an outbreak.

The experimental vaccine, manufactured by GlaxoSmithKline PLC, seems to work at low enough doses to overcome a key obstacle in earlier vaccines, which required high doses and would have been too cumbersome to produce in large quantities rapidly.

“It’s really taking us a quantum leap forward in terms of the feasibility of making a vaccine for people who need it in the case of an outbreak,” said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

GlaxoSmithKline said it had tested its vaccine in Belgium on 400 healthy people, ages 18 to 60, who were then given blood tests to measure their immune systems’ response. The tests showed that more than 80 percent of the subjects were protected by two shots, each containing only 3.8 micrograms of an antigen, an immunity-stimulating substance made from the bird flu virus.

By contrast, the first Sanofi vaccine protected only about 50 percent of the test subjects, who received two shots with much higher doses, 90 micrograms each.

The GlaxoSmithKline vaccine is not yet available, but the company expects to seek approval for it this year from the Food and Drug Administration and from drug agencies in other countries. The vaccine will sell for the same price as a standard flu shot, said Patty Seif, a GlaxoSmithKline spokeswoman. Worldwide, that cost for consumers averages $8 to $12 a shot, she said.

The bird flu virus spreading around the world, known as A(H5N1), so far has not managed to spread easily to people or between them. If it were to mutate in a way that made it more contagious among people, a deadly pandemic could erupt.

Since 2003, 232 people in 10 countries have contracted bird flu, and 134 have died. The worst current outbreak is in Indonesia, where many thousands of birds have been infected, 54 people have contracted the disease, and 42 have died.

In May, the U.S. government awarded more than $1 billion to five companies, including Glaxo, which are developing faster ways to mass produce vaccine in case of a pandemic.