Nearly a decade after a new chicken pox vaccine began driving down the number of children getting the itchy red spots, the federal government now has an ambitious goal: to try to eliminate the disease from this country.

Health officials want chicken pox, which afflicts an estimated 600,000 people a year, to go the way of other illnesses such as measles and mumps that were once a routine part of childhood.

But to get there, it might mean giving children a second shot of the chicken pox vaccine.

That would be good news for the vaccine’s maker, Merck & Co. But some doctors say it’s not clear whether it would be worth the cost and effort involved in adding another shot to a crowded lineup of childhood immunizations.

A large federally funded study in Philadelphia is allowing health officials to measure the effectiveness of the one-dose regimen, and decide whether a second dose is warranted.

“We would like to try to eliminate varicella,” the virus that causes chicken pox, said Dr. Jane Seward, head of the viral vaccine branch at the federal Centers for Disease Control and Prevention. “The basic goal of any program should be decreasing disease to the lowest extent possible if that’s not an undue burden on society.”

Introduced in 1995, the vaccine was not intended to wipe out chicken pox, but to significantly lessen the toll of the disease.

About 4 million people a year in this country, predominantly children, used to get chicken pox, a highly infectious viral disease that spreads from person to person by direct contact or through airborne droplets from coughing and sneezing.

Most weather the discomfort – fever and sometimes hundreds of blisters that scab over – without much trouble. But about 100 people died and 11,000 were hospitalized annually with complications such as pneumonia and encephalitis, a potentially deadly brain inflammation.

Right now children receive a single dose of vaccine between the ages of 1 year and 18 months. But some children who get the shot come down with so-called breakthrough cases of chicken pox that are usually milder.

“It’s been a very successful program to date,” Seward said. “But can we do even better? … Ten years into the program is it still acceptable for children to get 20 lesions and miss three or four days of school?”

CDC officials decided in the fall to pursue the goal of eliminating the transmission of chicken pox in the U.S. It would still crop up from time to time, in the same way measles do, but “you don’t have a case going to a case going to a case,” Seward said.

The CDC is doing a cost-benefit analysis of a second shot, she said, but no decision has been made on whether it is warranted.

Adding another shot to the lineup of childhood immunizations would not be done lightly, in part because children already get as many as 22 shots by the age of 2.

Money is also an issue. The chicken pox vaccine used in this country, Merck’s Varivax, has a wholesale price of $65.09 per dose, though it usually sells for less.

Dr. Robert Baltimore, a Yale University pediatrician and infectious disease specialist who serves on the American Academy of Pediatrics’ infectious disease committee, said the question that needs to be considered is whether a second dose would provide “sufficient impact to have a public health benefit.”

“It’s not good for kids to get sick,” he said. But on the other hand, there is cost and office time involved in giving another shot, and there may be “a limit on the number of vaccines the public is willing to accept,” he said.

Dr. Mark Warner, of Drexel Hill Pediatrics, said that from his perspective the one-dose approach is very effective.

“I’m not seeing a whole lot of breakthrough cases, and the ones I do see are not so sick,” he said.

“It’s not like doctors are begging” to add another shot, Warner said.

Merck in August submitted an application to the Food and Drug Administration for a four-prong vaccine, called ProQuad, against measles, mumps, rubella and chicken pox. If the vaccine is approved, the decision to add a second dose of chicken pox vaccine could be easier since a second measles-mumps-rubella (MMR) shot is already given.

The Philadelphia study, in the meantime, is giving the CDC a wealth of data on the status of chicken pox in the vaccine era. A network of hospitals, doctors offices and other sites reports suspicious rashes to the city health department, which then investigates.

One evening in November, nurse Karl Heath went to the home of LaShona Moment in the city’s Wynnefield neighborhood. When her 8-year-old twins broke out in red spots in October, Moment suspected they had chicken pox, even though one had the vaccine and the other had the disease as a baby.

“We want to know if children who got the vaccine are getting chicken pox,” said Heath, who pricked the index fingers of Malik and his sister Maliyah.

It’s not always easy to identify chicken pox since a mild case can resemble bug bites. As it turned out, the repeat blood tests on the Moment twins ruled out chicken pox.

The Philadelphia study shows that vaccinated children still have about a 2 percent chance per year of getting chicken pox, though these breakthrough cases have far fewer lesions, said Dr. Barbara Watson, medical specialist with the city’s immunization program.

The study is also documenting the steep decline of the disease. In 1995, there were 1,197 reported cases of chicken pox in west Philadelphia; in 2003, there were 130.

This time of year is the start of chicken pox season. The health department is investigating nine cases at a northeast Philadelphia school – a mere shadow of former outbreaks.

“In the old days, you’d have 600 kids out of school at any point during an outbreak,” said Watson, a consultant for Merck.

Still, not all children respond to the current vaccine. “With one dose, if you’re a vaccinated child in school and have very close exposure, you have a one in five chance of developing disease,” Seward said. “We should anticipate that after two doses, that risk would be 7 percent.”

But even if a second dose of chicken pox vaccine is recommended, it doesn’t mean that the virus will fade away.

After the active stage of infection, the chicken pox virus, called varicella-zoster, lies dormant in nerves along the spinal cord and can reemerge years later as a painful rash called shingles. Shingles provide another route for the spread of chicken pox. A person with shingles can infect someone with chicken pox if they lack immunity to the virus.

Merck is testing a shingles vaccine that is aimed at preventing the rash in older people. The vaccine might by extension help lower the incidence of chicken pox because “it may be able to reduce shingles as a source of transmission of the varicella-zoster virus,” said Michael Oxman, an infectious disease doctor at the VA Medical Center in San Diego who is heading a nationwide trial of the shingles vaccine.

But, he said, as long as people harbor the virus, chicken pox can’t be completely wiped out.

Starting this year, the CDC is making chicken pox reportable, which will allow health officials to better track the disease’s remaining hold.

Watson said she senses a shift in parental attitudes since the chicken pox vaccine became available almost a decade ago.

Skeptical parents used to reason, “`I had chicken pox and I survived. What’s the big deal?’” she said. “Now there’s been enough education for parents to know that 100 kids dying and 11,000 being hospitalized is a big deal.”