Observational Study of Vaccine Efficacy 14 Years After Trial of Hepatitis B Vaccination in Gambian Children

Chronic infection with hepatitis B virus is a leading cause of death from cancer in Africa. One-quarter of the 60 million carriers die either of primary hepatocellular carcinoma or cirrhosis of the liver. Yet while hepatitis B vaccination is the simplest and most effective intervention to prevent mortality in adults both globally and in Africa, only one country in west Africa and two in southern Africa have a continuing vaccination program.

The Gambian program was started in 1986 as part of a 40-year trial to test the efficacy of hepatitis B vaccination in the prevention of hepatocellular carcinoma. Nine years later, vaccine efficacy against either infection or chronic carriage was 93 percent. A long-term study from Senegal, which was small due to major losses to follow up, reported efficacies nine to 12 years after vaccination of 63 percent for infection and 87 percent for chronic carriage. In the current study, researchers investigated vaccine efficacy against infection and protection against chronic carriage after 14 years.

By 1998, 1,041 young people had been vaccinated in the villages of Keneba and Manduar. Of study subjects, 232 were vaccinated at age 1-5; 225 at age 5-9; 220 at age 10-14; and 175 at age 15-19. Between 1984 and 1998, vaccination dramatically reduced hepatitis B infection in children from 48 percent to 11 percent in Keneba and from 80 percent to 15 percent in Manduar. The corresponding changes in hepatitis B surface antigen carriage were from 13 percent to 1 percent in Keneba and from 35 percent to 2 percent in Manduar. Vaccine efficacy against chronic carriage of hepatitis B virus was 94 percent, which did not vary significantly between the age groups. Efficacy against infection was 80 percent. This was significantly lower in the oldest age group (65 percent). Of the uninfected participants in this age group, 36 percent had no detectable hepatitis B virus surface antibody. Time since vaccination and a low peak antibody response were the most powerful risk factors for breakthrough infection. Low peak antibody response was also a risk factor for chronic carriage.

"We have shown that despite low antibody concentrations, vaccine efficacy against chronic carriage of hepatitis B virus is remarkably well maintained in adolescence," the authors wrote. "However, vaccine efficacy against infection was also well maintained as 37 percent of teenagers were infected and 36 percent of those who were uninfected had undetectable concentrations of anti-HBs."

"The role, if any, that sexual exposure plays in maintaining and boosting immunity remains to be defined and at present there are insufficient data to decide if a booster dose would be useful in teenagers in highly endemic areas where 15 percent of sexual partners may be chronic carriers of hepatitis B virus. In an area of low endemicity teenagers vaccinated in infancy may lose immunity because of lack of exposure, and a booster dose may be necessary at the onset of sexual maturity. In this setting it may be more sensible to deliver the primary course of vaccine in adolescence," the authors wrote.

"In conclusion, our long term study of hepatitis B vaccination in infancy in a country where the infection is endemic showed that vaccine efficacy against infection waned with time but efficacy against chronic infection remained high over 14 years," the researchers wrote.

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