CHICAGO - Barely a decade after American children started getting vaccinations for chickenpox, the once-universal disease of childhood has virtually disappeared from many parts of the country, according to a report released Tuesday.

Some parents still hesitate to get their children immunized for what they consider a low-risk illness, but many doctors say the vaccine has drastically cut back on children's sick days and reduced potentially fatal complications. In Illinois, which requires schoolchildren to be vaccinated before starting kindergarten, chickenpox cases have declined more than 95 percent since 1994, the year before the vaccine was introduced.

Deaths related to chickenpox have nearly vanished, with just 15 annual fatalities estimated nationwide, according to the paper in Tuesday's edition of the journal Pediatrics, which reviewed a decade of research on the vaccine's effects.

Not long ago, most children could relate to the miserable plight of Michael Anzalotti, an 18-month-old from Chicago's Northwest Side who caught the disease in May, just before an appointment to get immunized. Now doctors say the illness is so rare that many medical students no longer know how to diagnose it.

Michael's mother, Jennifer Anzalotti, was startled to look behind her son's ear and find the first pock, which had the telltale look of a dewdrop on a rose petal.

"I still don't know where he got it, to be honest," said Anzalotti, whose family kept Michael away from other children for more than two weeks to avoid passing on his illness.

Serious side effects of chickenpox are rarer than ever, but they are real, experts say. The illness can lead to skin infections, inflammation of the brain and pneumonia. Such side effects happen most often in otherwise healthy patients, studies have found.

Before the chickenpox vaccine, more than 10,000 people were hospitalized each year with such complications, and nearly half of them were children under 5. But the new report found that hospitalizations have decreased by more than 75 percent in the last decade, with the largest reductions among children.

"Sometimes people don't realize that chickenpox could be that severe," said Dr. Mona Marin, report co-author and a medical epidemiologist at the Centers for Disease Control and Prevention's division of viral diseases.

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LEERY PARENTS

Convincing parents of the need for the vaccine still can be difficult. Elizabeth Wright of Chicago said she complies with the state's vaccination requirements for her three children but is leery of getting them any shots beyond that, such as the relatively new vaccine for human papillomavirus.

Wright said she plans on sticking with the required one dose of chickenpox vaccine, even though the CDC recommended in 2006 that children get two doses to ensure protection.

"It's almost like we're doing this just to avoid having them getting itchy for a week," she said of the chickenpox vaccine. "If they don't have to have the vaccine, I'd rather they not have it."

Very few parents refuse only the chickenpox vaccine; typically the ones who avoid it are those who say no to all vaccines, said Dr. Paul Luning, chief medical officer of the PCC Community Wellness Center in Oak Park, Ill.

The new report found conflicting evidence for a unique concern about chickenpox vaccine: the idea that it could lead to an increase in shingles among older people.

Shingles is a painful skin rash most common in people older than 65, caused by a reactivation of the chickenpox virus that they contracted earlier in life. Some scientists believe being exposed to children with chickenpox can act as a sort of natural "booster shot" for older people, sparking their immune systems and reducing the risk that their virus will reappear as shingles.

In theory the chickenpox vaccine could be short-circuiting that natural protection by reducing the number of children with chickenpox, thereby removing the natural booster that older people would have received. By cutting down on disease in one group, the vaccine may indirectly boost a related disease among older people.

"It's a theoretical concern; it makes sense, but nobody knows," said Dr. Ben Katz, a specialist in pediatric infectious diseases at Children's Memorial Hospital in Chicago.

The CDC's Marin said two of the studies her team examined found that the chickenpox vaccine had boosted shingles rates, but two other studies found no effect.

In some ways the chickenpox vaccine may be more helpful now than when it was released in the 1990s, said Dr. Kenneth Polin of Town & Country Pediatrics in Chicago. That is because of the increase in drug-resistant staph infections, which can use chickenpox sores to gain entry into the body.

"When kids scratch a pox lesion, it can allow the bacteria to get into the skin," Polin said. "We want to do anything we can to reduce the likelihood of that."

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'CHICKENPOX PARTIES'

Some parents who are leery of the vaccine actually welcome contact with infected children in the form of "chickenpox parties," intended to spread the disease and give unvaccinated children protection from it in the future.

Such get-togethers may have made sense in the pre-vaccine era, when the disease seemed inevitable and evidence suggested that children often had milder infections than adults. But most doctors strongly advise against such parties now, citing the potential for hospitalization and even death.

"It's really an archaic kind of thing," said Dr. Jaye Schreier, a pediatrician at Advocate Lutheran General Children's Hospital in Park Ridge, Ill. "We could also go down to the river with a washboard and clean our clothes that way, but let's not."

The rapid transformation of chickenpox into an unfamiliar, almost exotic disease has decreased familiarity of the illness among parents and even doctors. Several doctors said they now see perhaps two cases a year, making the diagnosis more challenging.

Fifty years ago, many mothers could diagnose diseases such as measles, whooping cough and chickenpox on their own because the diseases were so common, said Katz of Children's Memorial.

"Now we don't see those diseases anymore, so when they crop up, parents aren't as good at doing the diagnosis," he said. "They're just going to have to call us for that."