Are you sure? Challenge studies have been conducted in recent years for Norovirus[1] and Whooping Cough/Pertussis[2]. Admittedly, less is known about Coronavirus, but both those two can be dangerous.

1. https://www.ncbi.nlm.nih.gov/pubmed/25210140

2. https://clinicaltrials.gov/ct2/show/NCT03751514