Education has faced unprecedented disruption during the COVID-19 pandemic; evidence about the subsequent effect on children is of crucial importance. We use data from an oral reading fluency (ORF) assessment--a rapid assessment taking only a few minutes that measures a fundamental reading skill--to examine COVID's effects on children's reading ability during the pandemic in more than 100 U.S. school districts. Effects were pronounced, especially for Grades 2-3, but distinct across spring and fall 2020. While many students were not assessed in spring 2020, those who were seemed to have experienced relatively limited or no growth in ORF relative to gains observed in other years. In fall 2020, a far more representative set of students was observed. For those students, growth was more pronounced and seemed to approach levels observed in previous years. Worryingly, there were also signs of stratification such that students in lower-achieving districts may be falling further behind. However, at the level of individual students, those who were struggling with reading prior to the pandemic were not disproportionately impacted in terms of ORF growth. This data offers an important window onto how a foundational skill is being affected by COVID-19 and this approach can be used in the future to examine how student abilities recover as education enters a post-COVID paradigm.