Summer sale: Save 58%

Wen Shi Lee

Three vials with different vaccines against Covid-19 by (L-R) Moderna, Pfizer-BioNTech and AstraZeneca stand on a table. (THOMAS KIENZLE/AFP via Getty Images)

Which COVID vaccine is best? Here’s why that’s really hard to answer

Wen Shi Lee, Hyon Xhi Tan