The first COVID-19 vaccines will likely require two shots

Pfizer / BioNTech’s and Moderna’s COVID-19 vaccines have a lot in common: they’ve both been highly effective in clinical trials, they’re both gene-based vaccines, and they also each require two doses. That last point is pretty standard for vaccines. “If you look at all the FDA approved vaccines, the vast majority will require multiple doses,” says Otto Yang, an infectious disease specialist at UCLA Health. Here’s why some of the most promising coronavirus vaccine candidates are no exception.

A vaccine works by exposing the body to a small part of the virus so that the immune system can learn to recognize it. More than one dose means more opportunities for the immune system to figure out exactly how to counter a future infection. The...

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