Pfizer’s and BioNTech’s experimental COVID-19 vaccine BNT162b2 has been chosen for a phase 2/3 efficacy study of 9,000 participants following its success in a phase 1 trial.

BNT162b2 was selected for a late-stage trial over another candidate, BNT162b1, because the former “was associated with less systemic reactogenicity, particularly in older adults,” according to a non-peer-reviewed paper published online yesterday by medRxiv.

In the ongoing phase 1 U.S. study, the two vaccine candidates elicited antibodies that could neutralize the SARS-CoV-2 virus that causes COVID-19, at levels similar to or higher than those of people who were recovering from the infection on their own.