GENEVA, Nov. 2 (Xinhua) -- Genetic factors might influence human infection of bird flu, which may explain why some people get the disease and others don't, and why it remains rare, the World Health Organization (WHO) said on Thursday.

    Scientists suspect some people have "a genetic predisposition" for bird flu infection, and others don't, the UN agency said in a report, which generalized conclusions of a WHO expert meeting in September.

    The theory is based on data from rare instances of human-to-human transmission in genetically-related persons.

    "This possibility, if more fully explored, might help explain why human cases are relatively rare, and why the virus is not spreading easily from animals to humans or from human to human," the WHO said.

    The evidence to the theory is mainly from a family cluster of cases last May in North Sumatra, Indonesia, when seven people in an extended family died.

    Only blood relatives were infected in the Karo district of North Sumatra, the largest cluster known to date worldwide, "despite multiple opportunities for the virus to spread to spouses or into the general community," the WHO said.

    Bird flu has infected 256 people since late 2003, killing 152 of them, according to the WHO.

    Although it remains mainly an animal disease, experts fear the virus could mutate and spark a human influenza pandemic, which could kill millions.

    The present situation is still serious and the risk that a pandemic virus might emerge is not likely to diminish in the near future, the agency has warned.

    According to the WHO, the development of a pandemic vaccine has become more difficult following the divergence of circulating viruses into distinct genetic and antigenic groups.

    "To date, results from clinical trials of candidate pandemic vaccines have not been promising, as these vaccines confer little protection across the different genetic groups," it said in the report.

    International standards, or "benchmarks", for evaluating the efficacy of vaccines are urgently needed, and integrated studies of sera from individuals being vaccinated in various clinical trials would be equally useful - for industry as well as for national authorities, it added.