Predicting and comparing long-term measles antibody profiles of different immunization policies

Objective Measles outbreaks are infrequent and localized in areas with high coverage of measles vaccine. The need is to assess long-term effectiveness of coverage. Since 1991, no measles epidemic affecting the whole island ha s occurred in Taiwan, China. Epidemiological models are developed to predic t the long-term measles antibody profiles and compare the merits of differe nt immunization policies on the island. Methods The current measles immunization policy in Taiwan, China, is 1 dose of measles vaccine at 9 months of age and 1 dose of measles, mumps and rub ella (MMR) vaccine at 15 months of age, plus a 'mop-up' of MMR-unvaccinated schoolchildren at 6 years of age. Refinements involve a change to a two-do se strategy. Five scenarios based on different vaccination strategies are c ompared. The models are analysed using Microsoft Excel. Findings First, making the assumption that measles vaccine-induced immunity will not wane, the predicted measles IgG seroprevalences in preschool chil dren range from? 81% (lower bound) to 94% (upper bound) and in schoolchildr en reach 97-98% in all strategy scenarios. Results are dependent on the association of vaccine coverage between the fi rst and second dose of vaccine. Second, if it is assumed that vaccine-induc ed antibody titres decay, the long-term measles seroprevalence will depend on the initial titres post vaccination, decay rates of antibody titres and cut-off of seropositivity. Conclusion If MMR coverage at 12 months of age can reach: > 90%, it would b e worth changing the current policy to 2 doses at 12 months and 6 years of age to induce higher antibody titres. These epidemiological models could be applied wherever a similar stage of measles elimination has been reached.