Health officials and researchers from the United States Centers for Disease Control and Prevention say the decline shows that the benefits of the vaccine program - established in the 1990s - were larger than expected prior to its start.

According to a recent study, published in the August issue of medical journal, Pediatrics, between 2002 and 2007 the annual average number of chickenpox deaths in the U.S. was the lowest ever reported, with 14 deaths recorded in 2007 and just 13 the year prior, due to a particular vaccine. Chickenpox led to about 105 deaths a year during the pre-vaccine years of 1990 to 1994, researchers say.

Chickenpox is caused by a varicella zoster virus and produces fever and an itchy rash. In rare cases, it can be complicated by bacterial infections, swelling of the brain or pneumonia.

"This is one of our success stories," Dr. Charles Shubin, medical director of the Children's Health Center of Mercy FamilyCare in Baltimore, told U.S. News and World Report. "We don't know if immunization in childhood is going to make a difference in adult shingles because it hasn't been long enough," added Shubin, who is also an associate professor of pediatrics at the University of Maryland in Baltimore.

Before vaccination became mandated, a few million Americans caught the infection every year. The study found that the number of people who get infected has been cut dramatically, and although most cases were mild back then, thousands of people landed in the hospital due to the disease.

The CDC's new report from the National Center for Immunization and Respiratory Diseases, which updates an earlier analysis from 1995 to 2001, shows deaths have dropped by as much as 88 percent over the first 12 years in all age groups and by 97 percent in young people 20 and under, since the varicella vaccine was introduced.

Health officials, along with the American Academy of Pediatrics and the American Academy of Family Physicians recommend that children receive two doses - two shots - of a varicella vaccine, which is mandated in all states, and researchers say give better protection against the disease.

About a fifth of infants get some swelling and soreness around the injection site, however, fewer than one in 1,000 of those who get the shot end up developing a seizure from the fever, although experts say it's often harmless. Varivax, a varicella vaccine developed by Merck, costs about $84 per dose, but the new report says the vaccination program has exceeded expectations in terms of cost-effectiveness.

"Our analysis documents the impressive impact on varicella mortality of the US vaccination program, largely during a period when only 1 dose was administered," the researchers concluded in the study. "With the current 2-dose program, there is potential that these most severe outcomes of a vaccine-preventable disease could be eliminated."

Researchers are also making strides toward decreasing cases through vaccine initiatives for another virus, HIV/AIDS.

For more than a decade, the U.S. Agency for International Development has worked on AIDS vaccine development in other countries, with support from funders, particularly the federal government itself, and research partners to build and refine research capacity. The agency has conducted HIV-related research in Kenya, Uganda, Zambia, Rwanda, and South Africa.

"Our program has shown that given the right support, research labs in developing countries can work at standards as high as any you'll find in the U.S. and Europe," said Gwyneth Stevens, director of clinical laboratories in Africa for the International AIDS Vaccine Initiative, said in a written statement.

Researchers continue working on an AIDS vaccine and are said to have made significant strides in the past couple of years. In 2009, U.S. military and Thai researchers announced that a candidate vaccine regimen they had assessed in a large clinical trial provided 31 percent protection from HIV. There are an estimated 34 million people living with HIV and, each day, an additional 7,000 people are newly infected by the virus.