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?"

You can find Apoorva Mandavilli’s coverage of the coronavirus here.

For more on how our immune system fights viruses, check out our episode: How do flu vaccines work?

Find the rest of our episodes about coronavirus here, including episodes on masks, social distancing, and how we’re processing this historic time.

For more on how to know which sources of information are trustworthy, you can check out our four-part series Prove It:

Part one: A brief history of facts
Part two: Science under the microscope
Part three: The scoop on journalism
Part four: How to find the facts