An experimental vaccine for Lyme disease has shown promise in tests in mice, raising the possibility doctors could eventually inoculate people against the tick-borne disease, researchers reported.

The vaccine, made from a genetically engineered version of a protein from the organism that causes Lyme disease, appeared extremely effective at preventing mice from being infected with various strains of the Lyme bacterium."We're very encouraged," said Richard Flavell, a professor of immunobiology at the Yale University School of Medicine who helped conduct the research described Thursday in the journal Science.

Flavell cautioned, however, that much more research is necessary before researchers could even begin testing the vaccine on humans and predicted it would take five to eight years before it could be available to the public.

More than 21,000 cases of Lyme disease have been reported in the United States, and the number of cases has been increasing steadily over the past few years, with 7,402 cases reported in 1989, federal health officials said.

The disease is caused by a spiral-shaped organism known as Burrelia burgdorferi, a bacterium transmitted most commonly to humans through tick bites.

Although Lyme disease usually can be treated effectively with antibiotics, the illness can cause neurological damage, arthritis, heart damage and other serious complications if untreated.

Fort Dodge Laboratories in Fort Dodge, Iowa, began selling a Lyme disease vaccine for dogs in July. But that vaccine has not yet been tested on humans.

Flavell said he believes his vaccine may be safer because it uses only a single protein from the surface of the bacterium. The Fort Dodge vaccine is made with the whole organism after it has been killed, which could cause complications, Flavell said.