The federal government yesterday approved for sale the first genetically engineered vaccine for human use, ushering in a new generation of vaccines that might be safer to produce and might be used to prevent a variety of deadly diseases.

The Food and Drug Administration's action yesterday allowing Merck & Co. to sell its new vaccine against hepatitis B, "opens a new era in vaccine production," FDA Commissioner Frank Young said at a news conference yesterday.

"This opens the door for the production of other vaccines that have so far been impractical, potentially unsafe or impossible to make," Young said. Genetic-engineering technology, the manipulation of the genes of living cells, "holds great promise for future vaccines for AIDS or malaria, for example," he said.

Hepatitis B, a viral form of liver inflammation, is one of the most prevalent diseases in the world. An estimated 200 million people, or 5 percent, of the world's population are carriers, with the highest occurrence rates in Southeast Asia and sub-Saharan Africa. The disease infects about 200,000 Americans a year, causing fever, nausea, jaundice, cirrhosis of the liver and sometimes death. Some research also links it to liver cancer.

The disease is transmitted primarily by contact with contaminated blood and body fluids and has no cure.

A vaccine against hepatitis B has been available since 1981, but is derived from the blood plasma of patients infected with the disease. The process creates some risk for the technicians who draw and process the blood, and limits the potential supply. It also has created an unwarranted fear that the vaccine could cause Acquired Immune Deficiency Syndrome, because many carriers of hepatitis B are among groups at high risk for contracting AIDS, Young said.

The new vaccine is safer to produce, can be produced in greater quantity and should eliminate the AIDS-related fears because it is not derived from blood -- it is produced by genetically altered yeast.

A vaccine protects a person from a disease by stimulating the body's defenses. Conventional vaccines use a weakened or dead virus to make the body produce antibodies. The existing hepatitis B vaccine, also sold by Merck, uses parts of the virus that have been separated from the plasma.

The new vaccine also uses parts of the virus to stimulate antibody production, but produces those parts through genetically altered yeast. Scientists isolate certain genes from the virus and insert, or "splice," them into the genes of common brewer's yeast cells. The genes direct the cells to produce parts of the virus. The vaccine, taken through injection, uses those parts to trigger the production of antibodies that will attack the virus and kill it.

Gene-splicing enables the company to produce an unlimited supply of the product, whereas there are limited quantities of infected blood plasma. The genetically engineered form can be produced in a few weeks, while the plasma-derived product takes several months.

Researchers across the nation are using similar gene-splicing technology to try to develop an AIDS vaccine.

Chiron Corp., the genetic-engineering firm that developed the new vaccine Merck will market, is working on vaccines against oral and genital herpes, malaria and hepatitis A, a milder form of the disease.

Young urged individuals in high-risk groups to take advantage of the new vaccine, noting that only an estimated 3 percent to 30 percent of individuals at risk have done so. The high-risk groups, involving roughly 10 million people in the nation, include: medical and dental workers; homosexuals; drug abusers who use needles; immigrants from Asia; persons planning to travel to the Mideast, Asia or Africa; and babies born to mothers who are hepatitis B carriers.

Although fear of AIDS has been blamed as the major deterrent to vaccination, industry analysts say the vaccine's cost and method of delivery are as much a problem.

The new vaccine, available in January, will cost about the same as the old -- about $110 for a three-injection treatment, said Russell L. Durbin, a spokesman for Merck.

Industry analysts estimate the potential market for the new vaccine at $300-million-a-year worldwide, with about half the market in the United States.