Ever since antibody-dependent enhancement killed two toddlers in RSV vaccine trials in the mid-1960s it has been a front-of-mind consideration for all pharmaceutical companies developing vaccines. Because ADE manifests itself promptly upon a vaccinated host being exposed to infection it is relatively easy to spot; it generally manifests itself very early in the clinical trial process. And it stops the development of a vaccine dead in its tracks.
So far there have been over 4 billion doses of COVID-19 vaccines administered globally; documented cases of ADE is what? I'm not saying that there have been no cases but where's the data?