Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain
the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in
Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles
After a flurry of positive results from clinical trials of COVID-19 vaccines, developers are now seeking ‘emergency use’ approvals, which could see these immunizations deployed in potentially tens of millions of people. But scientists are concerned that this kind of early deployment could compromise the ongoing clinical trials that seek to show conclusively how well the vaccines work.