TUESDAY, Nov. 4 (HealthDay News) -- Too few American adolescents with asthma and other high-risk illnesses are getting flu shots, a new study finds.

Researchers analyzed vaccination rates from 1992 to 2002 for 18,703 adolescents with asthma, cardiac disease, immune system disorders and other conditions.

During the study period, vaccination rates improved, but only from 8 percent to 15 percent. From 1999 to 2002, only 11 percent of the patients received vaccinations during all four seasons, and more than 56 percent received no flu shots during those four years.

There were many missed opportunities, the researchers noted. From 45 percent to 55 percent of the adolescents who had one or more health-care visits during the flu season didn't receive a flu shot. Those who had preventive visits were more likely to receive influenza vaccine.

The findings were published in the November issue of Pediatrics.

"Influenza vaccination has been recommended for adolescents with high-risk conditions for well over a decade," study author Mari Nakamura, a clinical fellow in pediatrics at Children's Hospital Boston, said in a Harvard Medical School news release.

In children and adolescents with high-risk conditions, flu can lead to severe illness, hospitalization and even death.

Nakamura and colleagues concluded that both parents and health-care providers must be part of any intervention strategy designed to boost flu vaccination rates among high-risk adolescents. For example, letters to parents and electronic reminders to health-care providers have been shown to help improve vaccination rates.

"Our findings lend support for the simplicity of universal vaccination," Nakamura said. "More adolescents, especially those with high-risk conditions, may be vaccinated if providers and parents don't first have to identify who meets criteria for vaccination, as under a risk-based approach."

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