Analysis: What Lasting Academic (and Economic) Effects Could Coronavirus Shutdowns Have on This Generation of Students? Some Alarming Data Points From Research on Previous Disasters

...Researchers have attempted to quantify the loss of learning from school closures in a number of contexts. Natural disasters provide one useful parallel. For example, test scores fell in Thailand the year after severe flooding in 2011 forced schools in some provinces to close for up to a month. The losses were the most pronounced for younger students...

https://www.the74million.org/article/aldeman-what-lasting-academic-effects-will-coronavirus-school-shutdowns-have-on-students-research-on-previous-disasters-offers-some-clues/