* Esther Duflo and Lisa Ho: Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142 (email: eduflo@mit.edu; lisaho@mit.edu). We thank Angelica Chin, Jane Hu, Nic Minudri, Jose Angel Cazares Torres, and Elsa Trezeguet for their excellent research assistance. We are also very grateful to Dr. Sarah Liegl (St. Anthony North Family Medicine) and Dr. Susan Wootton (McGovern Medical School) as well as several doctors in the Assistance Publique/Hôpitaux de Paris (APHP), Paris hospital system who volunteered their time to create videos about COVID-19 vaccination. We thank Martin Hirsch, then director of APHP, for launching the study, and the entire team at APHP who made it possible (including Catherine Paugam, Patrick Chanson, and Sandra Fournier). We thank Elizabeth Bond and Amanda Girard for their excellent graphic design, and Matthew Wrather for creating the study websites. We thank Nisha Deolalikar for supporting the study at Facebook, which provided financial and logistical support by running ads for free and by hiring a marketing firm (Code3) to support the research team with the ad campaign. The NIH provided funding for the US part of the experiment under grant P30AG06419003 Sub: Project-001: 349273. APHP provided financial and logistical support for the French part of the study with administrative permissions and video production. We thank Rachel Glennerster, Amy Finkelstein, Ed Jee, and participants at the AEA meetings for comments and encouragement. The study received IRB approval from MIT (Protocols 2106000397 – US and 2111000508 France) and was registered in the AEA registry of social experiments (AEARCTR-0008711 – US and AEARCTR-0008902 France).