This story was originally published by Knowable Magazine.

It’s been a long year, but a vaccine against Covid-19 has started to roll out across the United States. There won’t be enough to vaccinate everyone right away, so public health officials will need to figure out how to manage the slow ramp-up of immunity.

Do we make faster progress by focusing on locked-down regions first, or on those that have tried to stay open? Dense urban areas or suburbs full of commuters? Should we skip over pockets with high levels of vaccine skepticism or push extra-hard to vaccinate there? And, ultimately, when will it be safe to relax, unmask, and start hugging people again?

Those are difficult questions to answer because the outcome depends on so many interconnected factors, including the unpredictability of individual humans’ minds and behavior. To make things even more challenging, human behavior is a moving target: What our leaders and friends say and do today influences what we ourselves do tomorrow. Effects can loop back to alter causes in a complex tangle that’s almost impossible for analysts to think through.

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