Influenza vaccination allocation in tropical settings under constrained resources.

UNLABELLED: Influenza virus seasonality, synchronicity, and vaccine supply differ substantially between temperate and tropical settings, and optimal vaccination strategy may differ on this basis. Most national vaccine recommendations focus on high-risk groups, elderly populations, and healthcare workers despite previous analyses demonstrating broad benefits to vaccinating younger high-contact age groups. Here, we parameterized an age-structured non-seasonal asynchronous epidemiological model of influenza virus transmission for a tropical low-income setting. We evaluated timing and age allocation of vaccines across vaccine supplies ranging from 10% to 90% using decade-based age groups. Year-round vaccination was beneficial when comparing to vaccination strategies focused on a particular time of year. When targeting a single age-group for vaccine prioritization, maximum vaccine allocation to the 10-19 high-contact age group minimized annual influenza mortality for all but one vaccine supply. When evaluating across all possible age allocations, optimal strategies always allocated a plurality of vaccines to school-age children (10-19). The converse however was not true as not all strategies allocating a plurality to children aged 10-19 minimized mortality. Allocating a high proportion of vaccine supply to the 10-19 age group is necessary but not sufficient to minimize annual mortality as distribution of remaining vaccine doses to other age groups also needs to be optimized. Strategies focusing on indirect benefits (vaccinating children) showed higher variance in mortality outcomes than strategies focusing on direct benefits (vaccinating the elderly). However, the indirect benefit approaches showed lower mean mortality and lower minimum mortality than vaccination focused on the elderly. SIGNIFICANCE STATEMENT: Influenza exhibits strong annual seasonality in temperate countries, but less consistent and predictable patterns in tropical countries. Many tropical countries are low-income countries with low influenza vaccine coverage. Globally, influenza vaccines are recommended for elderly adults and vulnerable groups, though evidence has shown that vaccinating school-age children is beneficial due to their high rates of social contact. Our modeling study evaluated whether age-based vaccine allocations can effectively minimize population influenza mortality in a tropical country with constrained resources and little seasonality. Prioritizing school-aged children for vaccination minimized mortality, with secondary emphasis on elderly adults. These benefits are most apparent under low vaccine supplies and can inform most effective ways to develop or expand influenza vaccination campaigns in low-income tropical settings.