Thus far, the data that’s been gathered on the COVID-19 vaccines suggests that the majority of reactions to the shots are mild — including things like headache, fever, fatigue and nausea. But with over 38 million people in the U.S. now fully vaccinated, the Centers for Disease Control and Prevention is continuing to gather information about how those who get the vaccines are responding, doing it through an — as of yet little-known — app called V-safe.

The CDC describes V-safe as a “smartphone-based tool that uses text messaging and web surveys to provide personalized health check-ins after you receive a COVID-19 vaccine.” The program asks you to report any side effects you experience after the vaccine and, depending on their severity, may prompt someone from the CDC to reach out for more information.

This week, the app received criticism for the fact that it’s available only on smartphones, excluding the millions of people in the U.S. who do not have one.

But for the vast majority who do, says Dr. Carolyn B. Bridges, associate director for adult immunization at the Immunization Action Coalition, it can be an extremely helpful resource. Here’s what you need to know.

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