Thank you too for commenting. Yes, swab test for Covid is highly accurate. But since the study only performed nasopharyngeal swab test of the upper respiratory tract, it may miss out on severe covid cases where the virus has progressed to the lower respiratory tract.

However, since the patients died unexpectedly, it's highly unlikely that there was covid progression from mild to severe, so nasopharyngeal swab may be enough to rule out Covid. Plus, the patients also had no prior history of Covid. I hope that makes sense.