Mary Duenwald, a Bloomberg Opinion editor, tested positive for Covid-19 even after receiving both doses of the Pfizer Inc.-BioNTech SE vaccine. Sam Fazeli, a pharmaceutical industry analyst for Bloomberg Intelligence, explains what happened. The reality is that so-called sterilizing immunity, or protection that completely blocks a virus from infecting you, is rare. In fact, only one vaccine has been proven to provide that, and that is the smallpox shot. If you think back to the phase III trials of the Pfizer-BioNTech and Moderna Inc. vaccines — arguably the ones with the best efficacy so far — they were 95% effective, meaning some vaccinated people still developed symptoms after the second dose, even if at a very low rate. Also, outside the environment of a clinical trial, efficacy will be slightly lower; real-world data is showing about 90% efficacy. It’s also possible that you are infected with a new variant, such as the one that is circulating in New York, known as B.1.526. These variants are potentially better at getting past your antibody immunity. Subscribe to our YouTube channel: https://bit.ly/2TwO8Gm Bloomberg Quicktake brings you live global news and original shows spanning business, technology, politics and culture. Make sense of the stories changing your business and your world. To watch complete coverage on Bloomberg Quicktake 24/7, visit http://www.bloomberg.com/qt/live, or watch on Apple TV, Roku, Samsung Smart TV, Fire TV and Android TV on the Bloomberg app. Have a story to tell? Fill out this survey for a chance to have it featured on Bloomberg Quicktake: https://cor.us/surveys/27AF30 Connect with us on… YouTube: https://www.youtube.com/user/Bloomberg Breaking News on YouTube: https://www.youtube.com/c/BloombergQuickTakeNews Twitter: https://twitter.com/quicktake Facebook: https://www.facebook.com/quicktake Instagram: https://www.instagram.com/quicktake