Why Does the AstraZeneca COVID-19 Vaccine’s Efficacy Vary So Much? Here’s What We Know

AstraZeneca's COVID-19 vaccine is up to 90% effective, the company said Monday in a press release, making it the third pharmaceutical firm to report promising vaccine news, following Pfizer and Moderna. AstraZeneca, which partnered with University of Oxford researchers to develop its two-dose vaccine, reported efficacy from two different dosing regimens; one led to 62% efficacy and another to 90%, with an average, the company says, of 70%.