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Why you can’t compare Covid-19 vaccines

What a vaccine’s “efficacy rate” actually means.

In the US, the first two available Covid-19 vaccines were those from Pfizer/BioNTech and Moderna. Both vaccines have very high “efficacy rates” of around 95 percent. But the third vaccine introduced in the US, from Johnson & Johnson, has a much lower efficacy rate: just 66 percent.

Look at those numbers next to each other, and it’s natural to conclude that one of them is considerably worse. Why settle for 66 percent when you can have 95 percent? But that isn’t the right way to understand a vaccine’s efficacy rate, or to even understand what a vaccine does. And public health experts say that if you really want to know which vaccine is the best one, efficacy isn’t actually the most important number at all.

Watch the video above to learn more about how these numbers were calculated and why the “best” vaccine is the first one you can get. And read more from Vox’s Umair Irfan on why these efficacy numbers can be misleading.

You can find this video and all of Vox’s videos on YouTube. Subscribe for more.