Infectious disease specialist explains why AstraZeneca's COVID-19 vaccine can vary in effectiveness

AstraZeneca announced Monday that its COVID-19 vaccine is up to 90% effective and even shows signs of reducing asymptomatic spread. It's effectiveness varied during clinical trials, depending on the dosage, but still averaged 70%. Dr. Uzma Syed, an infection disease specialist at New York's Good Samaritan Hospital Medical Center and director of Antimicrobial Stewardship Center of Excellence, joins CBSN's Elaine Quijano to discuss if the variation in effectiveness is something people should be concerned about.