An excellent review of hepatitis B virus (HBV) prevention through vaccination recently appeared in the New England Journal of Medicine (Dec. 30, 2004).
HBV is highly infectious in nonimmune persons. The incubation for acute infection varies between 45 and 160 days with an average of 90 days. Chronically infected humans are the prime reservoir for HBV. Acute HBV infection can be symptomatic or asymptomatic. Signs and symptoms are more common in adults than in children.
Acute disease eventually leads to viral clearance or to chronic infection. Chronic carriage can result in cirrhosis, liver failure, hepatocellular carcinoma, and death. Serious sequelae are greater if infection occurs during the first five years of life.
HBV infection occurs worldwide with many endemic areas. Most people are infected while very young. Up to 70 percent of adults in some areas experience infection. An estimated two billion people have evidence of an HBV infection with about 350 million chronic carriers. An estimated one million people die each year of HBV-related cirrhosis and hepatocellular carcinoma. HBV vaccination programs are now functioning in more than 100 countries. Adverse effects of chronic infection have decreased dramatically. The HBV vaccine is the first anticancer vaccine.
HBV infection most commonly occurs after percutaneous or mucosal exposure to blood or body fluids of infected people. Asymptomatic individuals can be infectious. Blood contains the highest titer of HBV. Common modes of exposure include sexual contact, contaminated needles, blood or blood products, and perinatal transmission. However, in a third of cases, the source of exposure is unknown.
The best way to avoid HBV infection is through immunization. Passive immunization involves application of hepatitis B immune globulin prior to or after exposure. Optimal protection entails active immunization, completing a series of three injections before infection occurs. Today, all vaccine formulations employ recombinant techniques where modified yeast cells generate the hepatitis B surface antigen protein, HBsAg.
HBV vaccine first became commercially available in the U.S. in 1981. The Advisory Committee on Immunization Practices recommended in 1991 a comprehensive strategy to eliminate HBV transmission, which included universal vaccination of infants. In 1995, a recommendation for routine immunization of adolescents was made. In 1999, the strategy was enlarged to include immunization of people up to age 18. The strategy proved successful. The annual incidence of acute HBV infections in the U.S. fell from 8.5 cases per 100,000 persons in 1990 to 2.8 per 100,000 in 2002, a decrease of more than 67 percent.
In 1991, the Occupational Safety and Health Administration started to require employers to inform at-risk employees of the vaccine, and to offer it free of charge. All current licensed HBV vaccines are administered intramuscularly. The usual immunization scheme for adults involves three doses - at zero, one and six months.
The antibody response to HBV immunization declines with a recipient’s increasing age. Nonresponse starts to decrease after age 30. Obesity, being male, and genetic factors also affect vaccination success. Immunization during childhood and adolescence offers the best chances for seroconversion and long-term, possibly lifelong immunity. More than 90 percent of all adults and 95 percent of infants, children, and adolescents produce protective levels of antibodies after vaccination.
In an ideal world, anti-HBsAg levels should be determined within one to three months after completing the vaccination series. Unfortunately, testing of antibody levels usually occurs years after immunization. A true nonresponse is quite different from a “waning” antibody level. Levels of antibody naturally decline over time, often to the point of being no longer detectable. In such cases, a single dose of vaccine is administered. This is followed by antibody testing four to 12 weeks later. No response means a lack of protection. Memory cells from an initially successful vaccination will produce an anamnestic antibody response, thus indicating protection.
For people who fail to respond after a booster dose, a second series of two injections will result in seroconversion rate of 50 percent to 60 percent among recipients. Often used is a double-sized vaccination dose.
OSAP, the Organization for Safety & Asepsis Procedures, is dentistry’s prime source for evidence-based information on infection control and prevention and human safety and health. Additional information concerning HBV transmission and HBV vaccination is available on the OSAP Web site - www.osap.org.
Dr. Charles John Palenik is an assistant director of Infection Control Research and Services at the Indiana University School of Dentistry. Dr. Palenik is the co-author of the popular “Infection Control and Management of Hazardous Materials for the Dental Team.” He serves on the Executive Board of OSAP. Questions about this article or any infection-control issue may be directed to firstname.lastname@example.org.