?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.au=Engzell%2C+Per&rft.aufirst=Per&rft.aulast=Engzell&rft.title=Proceedings+of+the+National+Academy+of+Sciences&rft.volume=118&rft.atitle=Learning+loss+due+to+school+closures+during+the+COVID-19+pandemic&rft.issue=17&rft.issn=0027-8424&rft.date=27+April+2021&rft.genre=article