By 2003, those states saw an 88% drop in
HAV incidence—to 2.5 cases per 100,000 population
from the average rate of 21.1 cases per 100,000
population recorded in the prevaccination period.10
The series of two HAV vaccinations administered over a sixmonth
period was shown to protect adults for at least 10 years and 99% of children for five to
six years.