How has the COVID-19 pandemic affected learning progress among school-age children? A growing number of studies address this question, but findings vary depending on context. We conduct a pre-registered systematic review, quality appraisal and meta-analysis of 42 studies across 15 countries to assess the magnitude of the effect of the pandemic on learning. We find a substantial overall learning deficit (Cohen’s d = -0.14, 95% c.i. -0.17, -0.10), which arose early in the pandemic and persists over time. Forgone learning is particularly large among children from low socio-economic backgrounds. It is also larger in math than in reading, and in middle-income countries, relative to high-income countries. There is a lack of evidence on learning progress during the pandemic in low-income countries. Future research should address this evidence gap and avoid the common risks of bias that we identify.