The Mathematical Model and the Vaccine Policy: Varicella and its Outbreaks in Maine
by
Jiancheng Huang
Maine Immunization Program, Division of Disease Control, State of Maine
Coauthors: Lisa Tuttle, Scott Goulette, Shawn Box, Jen Gunderman-King, Julie Keen, Paul Moffat, and Jiancheng Huang. Maine Immunization Program, Division of Disease Control, State of Maine

INTRODUCTION: Statistical techniques, including mathematical modeling, can play an important role in developing standardized clinical and public health guidelines and policies, especially when controversies exit. A mathematical model for optimizing vaccination of children with varicella vaccine was developed during the licensing process to address long-standing questions regarding varicella vaccine efficacy. It was a non-linear age-structured deterministic transmission model for varicella virus. The model demonstrated that the occurrence of varicella cases was very sensitive to the vaccine coverage rates and suggested the importance of a catch-up immunization program for the older children. This model was applied to the strategic development of Maine's varicella vaccine policy.

PURPOSES: (1) To determine the effectiveness of the varicella vaccine; (2) To demonstrate the importance of mathematical modeling in policy development; and (3) To evaluate the varicella vaccine policy by using data from varicella surveillance and outbreak investigations in Maine.

METHODS: To monitor varicella occurrence, a surveillance system based on school nurses'weekly reporting was set up in cooperation with the Maine Department of Education. The number of varicella cases at each school in Maine was reported through out the school year. To determine the effectiveness of the varicella vaccine, two outbreaks were extensively investigated during 2001 and 2002. Vaccine effectiveness (with 95% confidence interval) was calculated for each outbreak. Following the model, a policy of varicella vaccine for all school children has been installed in Maine.

RESULTS: More than 700 cases of varicella occurred in Maine during 2002-2003 school year. The cases more frequently occurred among young school children, especially among those who were unvaccinated. The effectiveness of the varicella vaccine was approximately 90% against all cases of varicella and above 95% against moderate and severe cases. The varicella events in Maine are within the range predicted by the mathematical model.

CONCLUSIONS: The varicella vaccine is highly effective. The occurrence of varicella cases and outbreaks in Maine reflect the fact that the vaccine coverage rate has not yet reached its optimal level. The goal of controlling varicella in Maine will be achieved by maintaining the current state varicella vaccine policy that was guided by the mathematical model.

Reference: Halloran ME, Cochi S, Lieu T, et al: Theoretical epidemiologic and morbidity effects of routine immunization preschool children with live-virus varicella vaccine in the U.S. Am J Epidemiol 140:81-104, 1994.

Date received: August 29, 2003