The COVID-19 pandemic is expected to have profound and enduring effects on mental health but, until we have data, we will not know its form, extent, duration or distribution. An appropriate public health response to mitigate and manage mental health sequelae is likely to require significant diversion of resources. In the post-COVID environment, such decisions must be underpinned by reliable information: policymakers, commissioners and services need to know both the scale of need and who is most vulnerable. A recent position paper in this journal1 highlights “an immediate priority is collecting high-quality data on the mental health effects of the COVID-19 pandemic across the whole population and vulnerable groups”. This should be a clarion-call for governments to fund, and researchers to gather, timely, high-quality population mental health data which represents the true need arising from the pandemic.