Shoulda mentioned this on the Assassins thread (https://sondheimforum.com/index.php?topic=1561.msg6150#msg6150) -- he revealed it (Weidman too) Monday night during Patrick Cassidy's reunion Zoomfest -- but here it is on BroadwayWorld:
https://www.broadwayworld.com/article/Broadway-Treasure-Stephen-Sondheim-Reveals-Hes-Fully-Vaccinated-20210309