The chicken pox vaccine has proved 85 percent effective — or about as good as experts expected — in preventing the childhood disease, according to the most rigorous study yet of its real-world use.

"It works under conditions of community use. Now everyone needs to use it, and chicken pox will become a thing of the past," said Dr. Jane Seward, who runs the chicken pox vaccine program at the Centers for Disease Control and Prevention.

The findings were published in Thursday's New England Journal of Medicine.

Chicken pox is usually mild in children. But it can open the door to pneumonia and other serious infections. Until recently, nearly everyone came down with the disease. Chicken pox was blamed for about 100 deaths a year in the United States. Each year brought about 4 million cases.

The injected vaccine — made from live but weakened copies of the virus — was introduced six years ago.

Since then, researchers at Yale and Columbia universities studied 591 children at 15 pediatric medical practices around New Haven, Conn.

The vaccine, which is recommended for most children 12 to 18 months old, was 85 percent effective overall. It worked especially well in warding off the more severe cases, where it was 97 percent effective.

"If its use is fairly widespread, the potential is there to totally eradicate the disease," said Yale's Marietta Vazquez, who led the study.

Tests before the vaccine won approval found similar rates of effectiveness. But researchers were not certain how well those findings would apply to broader use. For one thing, the vaccine is weaker than a major test version.

About 20 states already require chicken pox vaccinations for children starting school or day care. More than 60 percent of children are already given the vaccine, according to the CDC. It is aiming for 95 percent by age 5.

It remains unclear if patients are immune for life or need a booster dose. Studies over 10 and 20 years have found no breakdown in immunity against the normally once-in-a-lifetime disease.

Because chicken pox is often more serious in adulthood, some parents in the past tried to expose their children to the disease while they were young.

Seward said the vaccine may eventually prove useful also in fighting shingles, a disease caused by the same virus as chicken pox. Shingles cause a painful skin rash and sometimes chronic pain.