NEW YORK (Reuters Health) - Officials at the Centers for Disease Control and Prevention (CDC) said Thursday they are concerned that deaths due to chickenpox continue to occur in the U.S., despite the availability of a vaccine.

Preventing chickenpox-related deaths is a "public health priority," Dr. Mona Marin of the CDC's National Immunization Program said in a telephone interview with Reuters Health.

Chickenpox is caused by a virus called varicella, and a varicella vaccine has been available since 1995.

"Varicella is a vaccine-preventable disease, and we encourage physicians to use the vaccine because it was proven safe and effective in outbreak investigations," Marin said.

In the June 13th issue of its Morbidity and Mortality Weekly Report, the CDC describes two of nine fatal cases of varicella -- one in an adult and one in a child -- reported to the agency during 2002.

"Both patients were susceptible, unvaccinated, and exposed to unvaccinated children (with chickenpox)," Marin said. Both patients had also been previously healthy.

Importantly, Marin said, there continues to be "substantial underreporting" of varicella-related deaths, despite a mandate in 1999 that such deaths be reported to the National Immunization Program.

"We don't really know exactly how many varicella-related deaths are occurring," she said. "There may be many more."

For example, according to data from the National Center for Health Statistics for the year 2000, varicella was listed as the primary cause of death on 44 death certificates from 23 states and the District of Columbia, but only nine -- or 20 percent -- were reported to the CDC.

According to the agency, these chickenpox-related deaths emphasize the importance of recommendations for preventing infection, including routine vaccination at 12 to 18 months of age and vaccination of susceptible people in high-risk groups, such as child-care workers.

Marin said the CDC is "trying to implement school requirements for immunity to varicella, either by having had varicella or being vaccinated. This will decrease virus circulation."