Adequate preexposure vaccination for rabies allows persons who may later be exposed to rabies virus to be protected by receiving only two postexposure doses of vaccine. Although the World Health Organization and the Centers for Disease Control and Prevention recommend a three-dose preexposure series of vaccinations, countries such as Britain and France have introduced two-dose vaccination schedules. Strady and colleagues conducted a prospective study to determine the long-term immunity results of both two- and three-dose schedules for the two most commonly used rabies vaccines.

The study included 312 French volunteers who were at risk for rabies because of their profession. After screening for contraindications to vaccination, the participants were randomly assigned to receive either the human diploid cell rabies vaccine or the purified Vero cell rabies vaccine. Each vaccine group was further divided into a two-injection group or a three-injection group. Persons in the two-injection group were vaccinated on days zero and 28. The three-injection protocols included an additional vaccination on day seven. All patients received a booster vaccination at one year. Blood was drawn for antibody levels before the first injection and at day 42. Antibody levels were also checked one year after the primary series, 14 days following the booster dose and once per year for 10 years to assess the evolution of immunogenicity.

The only significant difference in the four study groups at baseline was the mean age of participants. However, this was not believed to influence the immunogenicity of the vaccines. All subjects seroconverted by day 42. The mean antibody titer was significantly higher in patients on the three-dose schedules than in those on the two-dose schedules. Both vaccines performed well when given in three-dose schedules, and there were no significant differences between the different vaccines in the three-injection schedule. When the two-dose schedules were compared, workers receiving purified Vero cell rabies vaccine had significantly lower mean antibody titers and more rapid decline in titers. Workers who received the three-injection schedules plus a booster at one year developed protective antibodies for at least 10 years. For both types of vaccines, the three-shot schedule plus a booster given at one year provided protection for up to 10 years in 96 percent of patients who were followed. Patients who did not develop such protection could be reliably identified by measurement of their antibody responses two weeks after the one-year booster injection (day 379).

The authors recommend that high-risk patients receive either type of rabies vaccination using a schedule of three injections plus a booster at one year. Antibody testing should be performed two weeks after the booster dose, and patients with antibody levels of less than 30 IU per mL should receive booster rabies vaccination every three years. High-risk patients with antibody levels of 30 IU per mL or greater following the booster dose may be revaccinated every 10 years.