COVID-19 provides an unprecedented opportunity to evaluate the contribution of public health and social measures (PHSMs) to controlling transmission and inform responses to future emerging respiratory pathogens. We analysed SARS-CoV-2 transmission across 126 countries spanning multiple epidemic waves and diverse policy environments during periods when PHSMs were the primary interventions available to influence the epidemic. By explicitly representing key epidemiological drivers of transmission and measuring reductions in residual transmission scaling after incorporating OxCGRT policy time-series data, we isolated the effect of PHSMs on transmission. PHSMs frequently explained a substantial proportion of the observed variation in transmission intensity, with particularly strong effects seen in many high-income countries. In countries with strong evidence for policy effects, reductions in transmission reached 60-80% or above, although the individual policies responsible for these effects varied considerably. Harmonised analyses considering the effect of community mobility showed comparably strong effects in a similar subset of countries.