Vaccines, when available, will likely become our best tool to control the COVID-19 pandemic . Even in the most optimistic scenarios, vaccine shortages will likely occur . Using an age-stratified mathematical model paired with optimization algorithms, we determined optimal vaccine allocation for four different metrics (deaths, symptomatic infections, and maximum non-ICU and ICU hospitalizations) under many scenarios . We find that a vaccine with effectiveness & #8805; 50% would be enough to substantially mitigate the ongoing pandemic, provided that a high percentage of the population is optimally vaccinated . When minimizing deaths, we find that for low vaccine effectiveness, irrespective of vaccination coverage, it is optimal to allocate vaccine to high-risk (older) age groups first . In contrast, for higher vaccine effectiveness, there is a switch to allocate vaccine to high-transmission (younger) age groups first for high vaccination coverage . While there are other societal and ethical considerations, this work can provide an evidence-based rationale for vaccine prioritization.