Largest study on masks and Covid-19 demonstrates their effectiveness in the real world December 2021 Author(s) Jason Abaluck, Laura H Kwong, Ashley Styczynski, Md. Ashraful Haque, Alamgir Kabir, Ellen Bates-Jeffries, Emily Crawford, Jade Benjamin-Chung, Salim Benhachmi, Shabib Raihan, Shadman Rahman, Neeti Zaman, Peter J. Winch, Md. Maqsud Hossain, Hasan Mahmud Reza, Stephen P. Luby, Abdullah All Jaber, Shawkee Gulshan Momen, Faika Laz Bani, Aura Rahman, Tahrima Saiha Huq & Mushfiq Mobarak