HOUSTON - Selectively targeting hepatitis A immunization (HepA) only for certain high-risk individuals has a minimal impact on the overall disease rate, according to information presented at the 49th Annual Meeting of the American Society of Tropical Medicine and Hygiene.
Approximately half of the reported hepatitis A virus (HAV) cases in the United States occur in people with no known risk factors, said Jay M. Lieberman, MD, chief, pediatric infectious diseases, Miller Children's Hospital, Long Beach, Calif., and associate professor of pediatrics, University of California Irvine.
"Even if you vaccinate all high-risk individuals, you'll still miss about 85% of the cases," he said.
New recommendations (as of 1999) suggest that children living in places where HAV rates are at least twice the national average (20 cases per 100,000 population) should be routinely vaccinated with HepA vaccine, and children living where rates are more than the national average (10:100,000), but lower than 20:100,000, should be considered for routine vaccination.
The likelihood of jaundice with HAV infection is related to age at infection. Fewer than 10% of children younger than 6 years develop jaundice, and more than 70% of infections are asymptomatic. In school-aged children, up to half have jaundice, said Lieberman, and in adolescents and young adults, 70% to 80% have jaundice.
Age-specific incidence rates show the disease peaks in children ages 5 to 14 years of age. One-third of all reported cases occur in children younger than 15 years. A substantial number of cases are reported in the 0- to 4-year age group, but since fewer than 10% of infected young children have jaundice, those cases represent just the tip of the iceberg of HAV infections.
"If you want to look at the infection rate in that age group, multiply the disease rate by about 10," said Lieberman.
Young children may be most susceptible since high concentrations of HAV are shed in the stools of infected individuals; 10,000 times the amount found in serum.
"If you were trying to devise a way to spread hepatitis A, you would create something called a day care center," said Lieberman. "It's the perfect environment for fecal-oral spread of the virus."
HepA vaccines are recommended only for children 2 years of age and older. Studies have shown that maternal antibodies interfere with the antibody response of infants given the HepA vaccine at 2 months of age. "We know maternal antibodies interfere with the response to some live viral vaccines, but these are inactivated vaccines, so this was unexpected," said Lieberman.
Lieberman said even with maternal antibody interference, infants vaccinated at 2 and 6 months responded well to a booster dose. "One possible way around [this] problem may be to give a booster dose later in life," said Lieberman.
He emphasized that further studies are needed to determine the youngest age at which HepA can be given safely.
For more information:
- Lieberman JM. Vaccination of children under 2. Symposium 1: HepA vaccination in children and foreign-born populations. Presented at the 49th Annual Meeting of the American Society of Tropical Medicine and Hygiene. Oct. 29- Nov. 2, 2000. Houston.
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