TOO MUCH OF A GOOD THING? WHEN TO STOP CATCH-UP VACCINATION

Purpose: “Catch-up vaccination” for age groups beyond infancy can be an attractive and effective means of immunizing people who were missed earlier, but the cost-effectiveness of these programs may diminish as infant vaccination increases and consequent disease prevalence decreases.  We develop a general framework for determining the optimal time to discontinue a catch-up vaccination program.

Methods: We present a general decision framework based on an age-structured epidemic model.  We consider the cost per quality-adjusted life year gained of catch-up vaccination efforts, as a function of infant immunization rates over time and consequent disease prevalence and incidence.  We develop a simplified version of the model that ignores future changes in disease incidence caused by catch-up vaccination. We illustrate our results with the example of hepatitis B catch-up vaccination in China.  We contrast results from the dynamic epidemic modeling approach with the simplified approach that ignores future changes in disease incidence. 

Results: As infant vaccination rates increase, it is cost-effective to discontinue catch-up vaccination sooner. For the case of hepatitis B catch-up vaccination in China, static models of disease may be appropriate for evaluating the cost-effectiveness of current catch-up vaccination programs.  The following figure shows that using a dynamic model does not substantially change the estimate of the value of a hepatitis B catch-up vaccination program for children aged 1-19 in China (top panel). Hepatitis B catch-up vaccination in China should be discontinued sooner for 12-year-olds than for 5-year-olds, and the time at which to discontinue such vaccination decreases as catch-up vaccination coverage increases (bottom panel).

Conclusions: Dynamic epidemiological models combined with cost-effectiveness analysis can be used to assess when to stop catch-up vaccination programs.  For diseases with a stable reservoir in the population where catch-up vaccination is unlikely to have significant effect on the overall burden of disease, relatively simple models that ignore disease incidence may suffice.  These models may also be simpler for decision makers to understand and implement than dynamic epidemic models because of lower data requirements.  In China, even with 90% infant vaccine coverage, hepatitis B catch-up vaccination may still be cost-effective many decades in the future.