SCHEDULING OF MEASLES VACCINATION CAMPAIGNS IN LOW-INCOME COUNTRIES: PROJECTIONS OF A DYNAMIC MODEL

Purpose: Measles continues to cause considerable disease burden in low-income countries: an estimated 23 million DALYs were incurred in 2001. Large-scale vaccination campaigns (SIAs) have dramatically reduced disease burden recently, but the optimal frequency of SIAs remains unknown. Our purpose was (1) to develop a dynamic model of measles transmission for estimating the optimal frequency of SIAs in low-income countries, and (2) to compare the predictions of the dynamic model to the equivalent static model. Methods: We developed an age-structured compartmental model of measles transmission in Cambodia, Ghana, India, Morocco, Nigeria, and Uganda. Compartments were introduced for maternally immune, susceptible, exposed, infectious and vaccinated individuals. The model was parameterized with country-specific demographic and vaccine coverage data, and region-specific attack rate and vaccine efficacy data. The model was validated against case reports from surveillance systems. We projected measles cases and deaths for SIA frequencies of every 4, 6, or 8 years, for a 20-year time horizon and under assumptions that the coverage rate for the first dose of routine measles vaccine (MCV1) improves at 0%, 1%, 2% or 3% annually. Results: In countries where MCV1 coverage has been below 65% (India, Nigeria), measles cases and deaths should remain low as long as SIAs are held every 4 years, but could increase dramatically once again if SIAs are held only every 6 or 8 years. For instance, the model projects an additional 3.8 million cases in Nigeria from 2008-2027 if SIAs are held every 6 years instead of every 4 years (assuming 3% annual improvement in MCV1). In the remaining countries (MCV1 coverage presently above 80%), absolute disease burden should remain low regardless of SIA frequency, due to herd immunity from strong MCV1 coverage. Results are qualitatively unaffected by assumptions about rate of improvement of MCV1 coverage. The equivalent static model predicts enormously higher disease burdens than the dynamic model, and does not match the observed data either qualitatively or quantitatively. This model could, and will, also be used to address issues of measles vaccine cost-effectiveness. Conclusions: The herd immunity effects captured by the dynamic model imply that the reduced measles disease burden seen in recent years can be sustained, as long as SIAs are held consistently every 4 years in countries with low MCV1 coverage.