What a vaccine's "efficacy rate" actually means.

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

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% when you can have 95%? But that isn't the right way to understand a vaccine's efficacy rate, or even to 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.

Further reading from Vox:

Why comparing Covid-19 vaccine efficacy numbers can be misleading: https://www.vox.com/22311625/covid-19-vaccine-efficacy-johnson-moderna-pfizer

The vaccine metric that matters more than efficacy: https://www.vox.com/22273502/covid-vaccines-pfizer-moderna-johnson-astrazeneca-efficacy-deaths

The limits of what vaccine efficacy numbers can tell us: https://www.vox.com/21575420/oxford-moderna-pfizer-covid-19-vaccine-trial-biontech-astrazeneca-results

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