Dr. Peter Hotez joins Chris Hayes to discuss the effectiveness of the Covid vaccine for people with compromised immune systems—and what the new CDC mask guidance means for them and their loved ones.  » Subscribe to MSNBC: http://on.msnbc.com/SubscribeTomsnbc MSNBC delivers breaking news, in-depth analysis of politics headlines, as well as commentary and informed perspectives. Find video clips and segments from The Rachel Maddow Show, Morning Joe, Meet the Press Daily, The Beat with Ari Melber, Deadline: White House with Nicolle Wallace, Hardball, All In, Last Word, 11th Hour, and more. Connect with MSNBC Online Visit msnbc.com: http://on.msnbc.com/Readmsnbc Subscribe to MSNBC Newsletter: http://http://MSNBC.com/NewslettersYouTube Find MSNBC on Facebook: http://on.msnbc.com/Likemsnbc Follow MSNBC on Twitter: http://on.msnbc.com/Followmsnbc Follow MSNBC on Instagram: http://on.msnbc.com/Instamsnbc