Less than half of U.S. teenagers were vaccinated against hepatitis A in 2009 – a rate that may leave a large portion of young people susceptible to hepatitis A infection in adulthood, a study has shown.

Nationally, the vaccination rate in 2009 was only 42% among 13- to 17-year-olds nationwide, although 70% of vaccinated teens had received the second dose necessary for durable immunity against hepatitis A. Just over half of vaccinated teens had received the vaccine before age 10 years, and 48% had been vaccinated at age 11 years or older.

In contrast, coverage rates in 2009 among teens for the tetanus toxoid–acellular pertussis vaccine and meningococcal conjugate vaccine were 55.6% and 53.6%, respectively.

The findings are from the first study to use provider-reported data to evaluate hepatitis A vaccination coverage.

Hepatitis A morbidity and mortality are more severe among adolescents and adults than children, noted Dr. Christina G. Dorell of the Centers for Disease Control and Prevention and her colleagues (Pediatrics 2012;129:213-21[doi: 10.1542/peds.2011-2197]).

Although hepatitis A infections are not common in the United States, 1,987 cases were reported nationally in 2009, with the highest rates of disease among young adults between 20 and 29 years of age. When unreported and asymptomatic cases are considered, the actual incidence of hepatitis A infections in 2009 was more than 10 times greater than the reported number, according to CDC estimates.

Dr. Dorell and her colleagues likened the potential threat of low hepatitis A coverage to that presented by measles outbreaks in undervaccinated population pockets.

To determine hepatitis A vaccination coverage rates, the researchers used data from the 2009 National Immunization Survey–Teen, a randomized telephone survey of parents that then sought additional data from vaccine providers identified by the parents. Nearly 35,000 parents completed telephone surveys, 20,066 of which could be matched with adequate provider information and entered into analysis.

Vaccine coverage varied widely depending on the adolescents’ state of residence. The 33 states (and the District of Columbia) that were the latest to adopt universal childhood hepatitis A vaccination recommendations (in 2006) saw the lowest rates of coverage as of 2009 – collectively, only 27.8%. However, those states also had the highest proportion of adolescents vaccinated at 11 years of age or older, likely because of recent adoption of the universal-vaccination recommendations.

In contrast, the 11 states that had adopted hepatitis requirements earlier (in 1999) saw nearly three-quarters (74.3%) of adolescents covered by one or more doses in 2009. The six states that in 1999 adopted a weaker recommendation to "consider" early childhood hepatitis A vaccination saw an overall vaccination rate of 54% in 2009.

Adjusted prevalence ratios of vaccination initiation were highest for states with a vaccination requirement and for adolescents whose providers recommended vaccination, the researchers found.