Philadelphia ? Chicken pox, once a rite of childhood, is headed for near-extinction, going the way of measles and mumps.

A study of chicken-pox cases in West Philadelphia and two communities in Texas and California found that a vaccine introduced in 1995 reduced cases of chicken pox by about 80 percent in six years, according to a study in Wednesday’s Journal of the American Medical Assn.

Researchers said they believe the viral illness, known for its itchy red spots, is steadily declining throughout the country.

Before the vaccine was introduced in 1995, about 4 million people in the United States, most of them children, got chicken pox each year, resulting in about 11,000 hospitalizations and 100 deaths annually. Late winter and spring are prime chicken-pox season.

Most children weathered the discomfort of the disease fever and sometimes hundreds of blisters that scabbed over without much problem, but the disease can have serious complications including skin infections, pneumonia and encephalitis, a life-threatening brain inflammation.

Chicken pox is still far from being wiped out, but the study illustrates the far-reaching effect a vaccine can have on the course of childhood illness.

“This is a real success story,” said Jane Seward, a vaccine expert who headed the study at the federal Centers for Disease Control and Prevention.

The study found that as more children became vaccinated, the number of chicken-pox cases plummeted, and fewer people were hospitalized with complications of the disease.

In West Philadelphia, the number of chicken-pox cases fell from 1,197 in 1995 to 250 in 2000, a decrease of 79 percent. Hospitalizations for chicken pox went from 20 in 1996 to 6 in 2000, said Barbara Watson, medical consultant to the Philadelphia Department of Public Health’s vaccination program, who directed the research here.

In Antelope Valley, part of Los Angeles County, the number of cases declined 71 percent during the time of the study. Travis County, Tex., which includes Austin, saw an 84 percent decrease.

The most marked declines were in children between the ages of 1 and 4, but the disease was down in all age groups of children and adults.

The study’s findings were so convincing, Seward said, that they should help put to rest any doubts people still have about the importance of the chicken-pox vaccine.

“The vaccine works, it’s safe, and this is the final piece of the puzzle. If you use it, the disease goes away,” Seward said.