What if they developed a life-saving vaccine and, relatively speaking, nobody came?

They did.

Hepatitis B, the most serious form of hepatitis, infects an estimated 200,000 Americans a year, sending about 10,000 to hospitals. As a major cause of cirrhosis and liver cancer, it kills about 5,000 a year in the United States and hundreds of thousands worldwide.

And it's preventable.

A safe, effective vaccine against hepatitis B has been available since 1982. But only one in nine Americans at high risk of the disease has received the vaccine. Many of those most likely to be exposed to hepatitis B -- health care workers, sexually active gay men, intravenous drug abusers, residents and staff of mental institutions and kidney dialysis patients -- are not even aware that a vaccine exists.

When the hepatitis B vaccine was introduced in 1982, many high-risk people avoided getting it because of rumors that it could cause acquired immune deficiency syndrome (AIDS). The fear arose because the vaccine is made from blood donations by people infected with hepatitis -- mostly gay men, who also account for about 70 percent of the reported cases of AIDS in this country.

That fear is unfounded, according to the federal Centers for Disease Control (CDC), because both the hepatitis and the AIDS viruses are completely deactivated in manufacturing the vaccine.

"Not only is there no evidence that it's a risk," said Dr. Peter Hawley, a microbiologist and medical director of the Whitman-Walker Clinic in the District, "but there's plenty of evidence that it couldn't possibly be a risk."

Another deterrent is cost. Vaccination against hepatitis B -- three shots over six months -- can cost $100 or more.

To encourage vaccination, the federal government and some other employers -- including some hospitals, prisons and institutions for the mentally retarded -- offer the shots free of charge to employes who are at high risk of being exposed to the hepatitis B virus.

But in the three years since the vaccine became available, only about 1 million Americans have received at least one dose, according to Merck Sharp & Dohme, the vaccine's only U.S. manufacturer. CDC estimates that about 8.7 million Americans are at high or moderate risk of exposure to hepatitis B.

"Cost is now the major problem," said Dr. Robert R. Redfield, an infectious disease investigator at the department of virus diseases at Walter Reed Army Institute of Research. Less than 10 percent of the workers at Walter Reed have been vaccinated, Redfield said.

Incidence of hepatitis B remains almost as high in the United States as before the vaccine was introduced, and it is epidemic in many parts of Africa and Asia, where it is the leading viral cause of cancer.

"The sad thing is that in 1985, we have the technology to prevent that spread of hepatitis B from happening, but in the world at large, we just can't afford it."

Two new factors could help overcome the cost problem: a new method of producing the vaccine through genetic engineering, and a new method of delivering the shots in smaller doses.

The existing Merck Sharp & Dohme vaccine, licensed by the Food and Drug Administration in 1981, is derived from plasma from blood donors who are carriers of the hepatitis B virus. But Merck and several other companies are developing hepatitis vaccines grown in yeast through recombinant DNA or genetic engineering technologies.

These recombinant vaccines are still undergoing clinical testing, but one or more is expected to receive FDA approval next year. Although the recombinant vaccine will be less expensive than the plasma vaccine, a spokesman for Merck warned that its research and development costs are substantial.

"There has been no price assigned yet to the recombinant vaccine," said Roy Walker, manager of communications for Merck. "But the expectation that the price would drop dramatically is probably misdirected."

Another potential cost-cutter involves delivering the vaccine in smaller doses intradermally -- by a shallow injection between skin layers -- instead of the usual method of injecting it deep into the muscle.

Redfield and other researchers at Walter Reed recently compared the two methods of injection and found "no significant difference" in their effectiveness. In 25 health care workers vaccinated intradermally, only one failed to develop immunity.

Since the intradermal dose was one tenth the intramuscular dose, it would provide "approximately a 90 percent reduction in the cost of the vaccine," the Walter Reed group claimed in the current Journal of the American Medical Association.

In an accompanying editorial, two doctors from Northwestern University estimated that intradermal vaccination could reduce the cost of a dose by 70 percent, to between $30 and $45.

"If intradermal immunization turns out to provide equal protection at less cost," Dr. Robert Craig and Dr. Charles Clayman said in the editorial, "it is imperative that the cost saving be passed on to the consumer. The full potential among high-risk individuals and Third World populations of hepatitis B vaccine may now be achievable."

Intradermal immunization has been available for decades, Redfield said, but until recently there was little need for an alternative to intramuscular immunization. The new incentive is cost. Many of the newer vaccines, including the one against hepatitis B, are much more expensive than, say, a tetanus shot.

"The easy vaccines are all made," Redfield said. "The tough ones are coming down the line. The pennies-per-dose vaccines have all been found."

"If it can be given intradermally, it would be a tremendous saving," said Dr. Saul Krugman, professor of pediatrics at New York University School of Medicine, whose research in the early 1970s was instrumental in development of the vaccine. "The problem is, that's not easy to do out in the real world."

Injecting a vaccine between layers of skin is technically more difficult, Krugman said. And Merck's own studies suggest that it is slightly less effective and more likely to cause a local reaction, a company spokesman said.

But Redfield said intradermal injection is effective and poses no particular difficulty for properly trained health care workers using small, specially designed intradermal needles.

Health authorities agree that a successful vaccination campaign also depends on educating the public about the risks of hepatitis B.

Many health care workers who are desperately afraid of contracting AIDS on the job, says Whitman-Walker's Hawley, are surprisingly unconcerned about the real and well documented risk of hepatitis. While there is no documented case of AIDS contracted by a health care worker on the job, health care workers die of hepatitis every year.

Hawley spoke recently at a local hospital to a group of about 30 nurses concerned about AIDS. The hospital had offered the hepatitis vaccine free, and Hawley asked how many had been vaccinated against hepatitis.

Only two raised their hands.