When planes fly from east to west, they are flying against a river of air called a jet stream. These air currents can make your flight longer or shorter, depending on which way you are going.
Over the approaching holidays, people around the world will want to travel to see friends and family. Getting tested for the coronavirus can make this safer, but testing alone is not a perfect answer.
The cold supply chain keeps vaccines fresh during distribution, but the current system is nowhere near large enough to distribute the billions of COVID-19 vaccines that the world needs.