WASHINGTON (AP) -- Vaccines can keep chickens from dying of bird flu, but can immunized birds still silently spread infection?It's an important question, as China and Vietnam vaccinate millions of chickens in an effort to stamp out a worrisome strain of bird flu called H5N1.Scientists in the Netherlands put the question to a test -- using vaccines against a different strain -- and concluded that vaccinating poultry indeed can block viral spread between birds."Vaccination can be an attractive tool to prevent outbreaks of highly pathogenic AI (avian influenza) viruses in poultry, thereby achieving the aim of eliminating the source of human infections," concludes lead researcher J.A. van der Groot of the Netherlands' Central Institute for Animal Disease Control.Birds catch numerous strains of influenza, but only a few types are particularly deadly to both fowl and people. Today, the H5N1 strain is the worst. At least 68 people so far have died from H5N1 in Asia since 2003, almost all linked to contact with sick birds -- and millions of birds have died or been slaughtered in an effort to contain the virus. Health experts fear that the bird flu one day could mutate into a virus that is easily passed from person to person, sparking a global epidemic.Hence the interest in chicken vaccines.Previous research found that vaccination could protect individual chickens from falling ill with various flu strains. But there have been reports of asymptomatic chickens shedding virus after vaccination, raising concern.So van der Groot and colleagues tested two vaccines against the H7N7 bird-flu strain, by housing infected chickens together with healthy vaccinated ones.Two weeks after inoculation, both vaccines completely blocked H7N7 spread between chickens, they reported Monday in Proceedings of the National Academy of Sciences.Some transmission occurred one week after vaccination, but the virus' reproduction rate already had dropped enough that the researchers predict only a small number of new infections that soon after the shots.There were marginal differences in effectiveness between the two vaccines, however, leading the researchers to conclude that poultry vaccines' ability to stop viral spread should be tested before health authorities choose which one to use.The research backs guidelines from the United Nations' Food and Agriculture Organization, which recommends targeted vaccination of poultry as one measure to control outbreaks of bird flu -- along with other steps such as culling infected flocks -- and already warns that it can take two weeks for full protection.So far, the H5N1 virus has not been found in U.S. birds. If a bird outbreak did occur here, the Agriculture Department stockpiles poultry vaccine that would be used to help contain it.Copyright 2005 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.