WASHINGTON - Government scientists say they have successfully tested in humans a vaccine that they believe can protect against the strain of avian influenza that is spreading in birds through Asia and Russia.
Health officials have been racing to develop a vaccine because they worry that if that strain mutates and combines with a human influenza virus to create a new virus, it could spread rapidly through the world. (The vaccine cannot lead to such a situation because it is made from killed virus.)
The major issue now will be producing enough of the vaccine to meet the widespread demand in the event of a pandemic, said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases.
"The good news is that we have a vaccine now," Fauci said last night. "The sobering issue is that we still in this country, and in the world, do not have a good capacity to make vaccines."
Tens of millions of birds have died after being infected with the avian flu or after being culled to prevent the spread of the virus. About 100 people have been infected, and about 50 have died of this strain of the avian influenza virus, called A(H5N1). There has been no sustained human-to-human transmission, but that is what health officials fear, because it could cause a pandemic. And that fear has driven the intense research to develop a vaccine.
Fauci said that though the vaccine that has undergone preliminary tests - including some in Maryland - could be used on an emergency basis if a pandemic developed, it would be several months before that vaccine is tested further and, if licensed, offered to the public.
The American vaccine is derived from a strain that Dr. Robert G. Webster helped isolate from a human case in Vietnam. He is an internationally known influenza expert at St. Jude's Children's Research Hospital in Memphis, Tenn., whose work is supported in part by Fauci's institute.
The University of Maryland School of Medicine in Baltimore is one of three sites around the country where the vaccine is being tested.
In April, the school administered the first of two injections of the vaccine to 150 healthy people between the ages of 18 and 65. Trials were also administered at the University of Rochester in New York and the University of California, Los Angeles, with 452 volunteers in all, Fauci said.
Doctors at the test centers drew blood from the volunteers to document that they had no antibody to A(H5N1) before they received the first vaccine injection. Then the doctors drew another blood sample four weeks later when the volunteer received a second injection of the vaccine.
Vic Maslanka, one of the volunteers in the Baltimore trial, said he is due back for more blood work in October. He called yesterday's announcement "a big breakthrough."
The 48-year-old engineer from Monrovia, near Frederick, said he has not had any side effects from his injections, though he does not know whether he received a placebo or, if he received the vaccine, the dosage.
Four different dosages were administered to the volunteers: 7.5 micrograms, 15 micrograms, 45 micrograms or 90 micrograms.
Fauci has said that tests have shown that the new vaccine produced a strong immune response among the volunteers, although the doses needed were higher than in the standard influenza vaccine offered each year.
Fauci said that, within a month, the vaccine will be tested on people ages 65 and over. After that, he said, it will be tested on children. He expressed confidence that they would confirm the success of the first tests and answer remaining scientific questions.
An earlier human vaccine against A(H5N1) avian influenza virus was prepared after it first appeared in the world, in Hong Kong in 1997. That vaccine was never fully developed or used, and the strain has mutated since then.
Because the vaccine is made in chicken eggs, "a potential major stumbling block" to successful mass production is the number of eggs farmers can supply manufacturers, Fauci said.
If manufacturers can overcome such hurdles, the new vaccine could go far in averting a possible pandemic of human influenza, he said.
Only a small number of human cases of A(H5N1) influenza have been found. Although a few cases might have been transmitted from person to person in Asia, the A(H5N1) strain has not garnered enough strength to spread widely among humans anywhere.
As of Friday night, according to the World Health Organization in Geneva, the avian strain has killed 57 of the 112 people it has been known to infect in four countries. Those countries are Cambodia (four cases), Indonesia (one case), Thailand (17 cases), and Vietnam (90 cases).
Although cautioning that the vaccine has not been fully tested, Fauci said that the initial test findings have given the federal government enough confidence to start the process of adding tens of millions more doses of the vaccine to the 2 million it has bought. The present supply is stored in bulk form, and "we cannot put it in vials until we find out what the right dose is," Fauci said.
The manufacturer needs to know the dose and regimen to determine how much more vaccine it can produce and make available to the United States and other customers.
Evidence that the vaccine could be protective came from analysis of 113 of the 452 participants in the study. These findings have been discussed only among the doctors who conducted the studies and the scientists at his institute, Fauci said.
Though the numbers were small, they were large enough to show scientifically that the vaccine elicited a strong immune reaction, a critical point of the study.
The only way scientists can prove the effectiveness of the vaccine is if an epidemic of the A(H5N1) strain occurs among people. Ethics prohibit deliberately infecting people for experimental purposes, in part because that step could lead to the kind of pandemic that scientists are trying to prevent.
Under such circumstances, scientists must show that as the dose of the vaccine is progressively increased, the immune response increases proportionately.
Even when additional tests are completed, more time will be needed before the Food and Drug Administration can license the human vaccine and before policymakers determine when and how it should be administered.
The New York Times and Sun staff writer Sara Neufeld contributed to this article.