Survivors of severe COVID-19 may be seeing long-term symptoms ease after vaccine Experts are yet unsure why some COVID-19 survivors have PASC and why others don't, but post-viral syndromes are not new: After the 1918 flu pandemic, some survivors of that flu developed Parkinson's. and last updated 2021-04-26 17:56:07-04 Post acute sequelae of COVID-19, or PASC, is the term to describe survivors of severe COVID-19 who suffer long-term symptoms like chronic fatigue or a loss of taste and smell longer than four weeks after recovery. Experts are yet unsure why some COVID-19 survivors have PASC and why others don't, but post-viral syndromes are not new: After the 1918 flu pandemic, some survivors of that flu developed Parkinson's.