The findings published Thursday in New England Journal of Medicine found the vaccine protects only about half the people who receive it -- and then only when given high doses, reports The New York Times.
The researchers say manufacturers may not be able to ramp up output in a pandemic situation.
The study said a dose 12 times the amount used in a standard flu shot protected 54 percent of the people. An editorial by Dr. Gregory A. Poland of the Mayor Clinic which accompanied the findings in the medical journal said that level is "poor to moderate at best."
Dr. Anthony Fauci with the National Institute of Allergy and Infectious Disease, which paid for the development and testing of the vaccine, said: "We have a long way to go," adding the research is a step in the right direction, The Times report said.
Separately, the Wall Street Journal reported that vaccine production currently is restrained by the limited capacity of the makers, who also must grow the vaccine's virus particles in fertilized hen eggs.
Future research may include adding an immune system booster to the vaccine to try to make it work better at lower doses.