Since the U.S. began routine chickenpox vaccination in 1995, the number of Americans sent to hospitals by the infection each year has dropped by more than two-thirds, government researchers reported.

Studies have shown that after vaccination against the varicella virus that causes chickenpox became standard, yearly rates of chickenpox infections in the U.S. fell by 80 percent to 90 percent over the next decade.

For the new study, researchers at the Centers for Disease Control and Prevention (CDC) looked at how rates of chickenpox-related hospitalizations have changed.

While most cases of chickenpox simply make people feel miserable -- with symptoms including an itchy, blister-like rash, fever, headache and fatigue -- some people do develop potentially serious complications.

They include skin infections, vomiting that leads to dehydration, pneumonia and inflammation of the brain known as encephalitis.

The CDC researchers found that between 2000 and 2006, the yearly number of hospitalizations for chickenpox complications stood at 0.1 for every 10,000 (or one in 100,000) Americans. That compared with a rate of 0.4 per 10,000 people each year between 1988 and 1995 -- before the varicella vaccine was introduced.

Overall, hospitalizations fell by 71 percent over the study period. The researchers estimate that chickenpox vaccination prevented a total of 50,000 hospitalizations between 2000 and 2006.

"This further supports what we've been seeing -- in that there have been great declines in severe (chickenpox) disease," said lead researcher Adriana S. Lopez.

In an interview, Lopez noted that many people see chickenpox as a mild illness. But, she said, parents should be aware that complications can occur.

She added that even though hospitalizations are down, children age 4 and younger still have the highest rates of illness severe enough to require hospital care -- at 0.7 per 10,000 children between 2000 and 2006.

"It's still important for parents to have their children vaccinated," Lopez said.

During the period Lopez and her colleagues studied, children routinely received one dose of the varicella vaccine between the ages of 12 months and 18 months. In 2007, a second dose was added to the schedule, to be given between the ages of 4 and 6.

Adults and teenagers who had already gotten the vaccine were also advised to get the second dose. (While chickenpox most often infects children, the vaccine is recommended for teens and adults who've never had chickenpox and therefore lack natural immunity.)

The second dose was added because one dose of the vaccine prevents chickenpox in only 80 percent to 90 percent of recipients -- although people who are infected after getting just one vaccine dose usually have only a mild case of the disease.

An important finding of the current study, according to Lopez's team, is that chickenpox hospitalizations declined by more than two-thirds in each age group the researchers considered, including adults.

"It's great to see that in adults," Lopez said, noting that this is a sign of "herd immunity" -- when widespread vaccination in a population helps limit disease transmission, preventing infections even in people who are not immune.

The varicella vaccine can have side effects.

But according to the CDC, they are usually mild: about 20 percent of people have swelling or soreness at the injection site, and up to four percent develop a mild rash and some skin bumps that resemble chickenpox.

Serious side effects, the agency says, are rare -- rare enough that it is not clear whether the vaccine itself is always responsible. Fewer than one in 1,000 recipients suffer fever-induced seizures, and problems like encephalitis and pneumonia have been reported for about two of every 100,000 doses of the vaccine.

"This is a very safe vaccine," Lopez said.

The current study did not look at the cost-effectiveness of the chickenpox vaccination program. The vaccine, made by Merck & Co., costs consumers about $84 per dose.

In a 2008 study, however, the CDC estimated that widespread vaccination is saving money when all "societal" costs are considered. That includes doctor visits and other medical expenses, plus costs like parents' time lost from work and spending by local health departments to manage chickenpox outbreaks.
The CDC estimated that for every dollar spent on the vaccination program, more than four are saved.