How Can You Tell If A COVID-19 Vaccine Is Working?

A scientist at work on a COVID-19 vaccine candidate at Bogazici University in Istanbul in August.

Several COVID-19 vaccine candidates are being tested now. But why does it take 30,000 volunteers to know if one is safe and effective? And what does it mean to say a vaccine candidate is working?

(Image credit: Onur Coban/Anadolu Agency via Getty Images)

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