How has the COVID-19 pandemic affected learning progress among school-age children? Two years after the onset of the pandemic, a growing number of studies have addressed this question, but findings vary depending on context. We conduct a pre-registered systematic review, quality appraisal and meta-analysis of 34 studies across 12 countries to assess the magnitude of the effect of the pandemic on learning and the role of different factors in moderating this effect. We find a significant overall learning deficit early in the pandemic (Cohen's d =-0.17, 95 c.i.-0.22,-0.13), which persists over time. Forgone learning was particularly large among children from low socioeconomic backgrounds and in contexts with high excess mortality, longer school closures, and less-developed digital learning infrastructures. Future research should assess the effects of the COVID-19 pandemic in low and middle-income countries, as existing studies mainly focus on high-income countries.