In certain states, particularly those in the south, midwest and eastern U.S., those rates are lower -- with about three kids out of 10 getting one shot and about two out of every 10 kids getting the full immunization.

"One reason for lower rates of hepatitis A vaccination in some states is because of the recommendation history," said Dr. Christina Dorell, the lead author of the study in the journal Pediatrics and a researcher at the CDC.

In 1999, the CDC's Advisory Committee on Immunization Practices recommended the vaccine for children in 11 states in the western United States where infection rates were the highest. In six other states, the Committee said that the vaccine could be "considered" for children.

In 33 states, recommendations for routine vaccination came on board in 2006, but only for one-year-olds. For teenagers, the guidance is less strong, again saying that the vaccine can be "considered" for them.

Hepatitis A infection causes inflammation in the liver, and can lead to fatigue, poor appetite, nausea and jaundice. The CDC estimates that about 21,000 new cases occur each year.

EARLIEST STATES HAVE HIGHEST RATES

To get a measure of how many teens have received the vaccine under these various recommendations, the researchers surveyed more than 20,000 parents across the country and checked the immunization records of the kids' doctors. All the kids were born between 1991 and 1997.

Among the first 11 states to receive the Committee's recommendation for routine vaccination, the vaccination rates were the highest in the country: 60 percent of kids had completed the two doses by age 13 to 17.

Alaska and Oklahoma had the highest rates, at about 85 percent.

Among these states, black, Hispanic, American Indian and Alaska Native children were more likely to have been vaccinated than white children.

Dorell said these groups were originally targeted to get the hepatitis A vaccine because they had the highest rates of infection.

"Greater efforts among these communities allow them to have higher coverage. So it looks like those groups who seemed to be most at risk are actually being vaccinated" the most, Dorell told Reuters Health.

In the six states that have had the recommendation since 1999 to consider vaccination, 39 percent of teens have received the full immunization.

"Because it doesn't say, 'routinely have all adolescents vaccinated,' there's some wiggle room," Dorell said.

As a result, the 33 states that have had the same "consideration" recommendation for teenagers for the shortest time period, since 2006, had the lowest number of teenagers being immunized: 16 percent got both shots.

South Carolina and Mississippi had the lowest rates of teen immunization.

"That should be much higher. It would be terrific if we could vaccinate every adolescent for hepatitis A," said Dr. Cynthia Rand, an assistant professor of pediatrics at the University of Rochester Medical Center.

HOW MANY NEED TO BE VACCINATED?

The vaccine costs $14 to $30 per dose. It's not clear how many teens would need to be vaccinated for the vast majority of them to be protected because of a concept known as herd immunity. That means some unvaccinated people are protected because the infection can't take hold in those vaccinated.

Most hepatitis A infections occur in young children, and studies have shown that infection rates decline in other children and adults as hepatitis A vaccination rates increase among children," Dorell said. There was a 92 percent decrease in hepatitis A rates from 1995 to 2008 because of vaccines, she said.

Dorell said it's not known what percentage of teens would be sufficient to provide near-universal protection, but added that if all children were vaccinated at the age of one, "this will help maintain the great strides that have been made in decreasing infection rates in the U.S. over the past two decades."

"Additionally, vaccinating adolescents will also allow for their protection, not only from young children, but other sources including contaminated food, international travel, and other routes," she said.

"I think the main barriers (to getting the vaccine) is that the disease is not considered a high risk disease, and there is no school requirement in our state and in most states," said Rand, who was not involved in this study.

In states that do require hepatitis A vaccine for schools or daycare, the researchers found that vaccination rates were considerably higher.

Also, kids whose doctors recommended the vaccine were more likely to be immunized.

"It's important to educate providers and parents about why hepatitis A vaccination is important," Dorell said.

Although the disease has the worst impact on the health of older adults, Rand said it's important for children to be immunized because they often spread the infection to more vulnerable people.

SOURCE: bit.ly/yM9eHQ Pediatrics, online January 23, 2012.

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