In December, people started getting vaccinated against COVID-19. This is a huge scientific accomplishment and important step in making it safe for us to hang out in person again. So how did scientists develop these vaccines so fast? And how did they test the vaccines to make sure they’re safe? And how do these mRNA vaccines work?We have answers to all your questions, plus New York Times science journalist Apoorva Mandavilli explains what scientists have found out about how long immunity lasts to this new coronavirus. And Kara and Gilly are back with a cow-side chat about herd immunity. Plus: A new mystery sound and a Moment of Um that answers the question, "What would happen if the moon fell down onto earth?"Today’s episode is sponsored by Sitka Salmon Shares (http://sitkasalmonshares.com/brainson)See omnystudio.com/listener for privacy information.