James Robertson, principal scientist of the virology division of the National Institutes for Biological Standards and Control in Britain, said scientists need to develop a vaccine that will be effective at lower doses.
Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Disease, announced the vaccine recently. The first trials on adults showed two doses, each six times as large as the single dose needed for ordinary flu vaccines, are needed for protection.
One difficulty is that people have no natural immunity to avian influenza as they do to other strains of flu. But some researchers say a low-dose vaccine could work if it is combined with a chemical known as an adjuvant that irritates the immune system.
Avian influenza has struck only about 100 people worldwide. Researchers fear a pandemic if the virus mutates so it spreads person to person.
U.S. health officials hope to stockpile enough vaccine for 20 million people. The manufacturer, Sanofi Pasteur, has said it could double its production capacity in the next few years.