KEYWORDS:
hepatitis A, mean, median, statistics, incidence

BACKGROUND:
Hepatitis A continues to be one of the most frequently reported vaccine-preventable diseases in Texas. The Advisory Committee on Immunization Practices (ACIP) recommends routine vaccination of children living in areas where the average annual hepatitis A rate during 1987-1997 was > 20. The Texas Department of Health passed a school rule that allows the Department to mandate hepatitis A vaccination in counties with high incidence of disease. The Centers for Disease Control and Prevention identified 34 counties as meeting these criteria, based on the 1987-1997 data. However, occasional “outbreak” situations were not taken into consideration.

OBJECTIVE(S):
To demonstrate the methodology Texas is using to identify high areas of incidence indicating the need for a school requirement for hepatitis A vaccine.

METHOD(S):
The Department will calculate the average incidence for a 10-year period. However, this 10-year period will not be constant, but will be calculated 10 years back from the end of a calendar year. In addition, the Department will take into consideration “outbreaks” that occur in sparsely populated counties. This will be accomplished by computing the median incidence, which is not drastically affected by extreme values, as is the mean. Vaccination will be required for those counties with a mean and median incidence > 20 per 100,000 population and will be recommended for counties with a mean incidence between 10 and 20 and a median incidence > 10.

RESULT(S):
Six counties, instead of 34, are identified as having a mean and median incidence greater than or equal to 20.

CONCLUSIONS(S):
Calculating the mean alone does not identify areas that truly need mandatory vaccination against hepatitis A. Using the median deals with the extreme values and allows the Department to use the resources most judiciously.

LEARNING OBJECTIVES:
Understand the difference between the mean and median.