The pandemic of the Spanish flu ended in around three years, so I did not think that this corona pandemic would last so long, especially in this age of advanced medicine.
ご質問ありがとうございます。
"The pandemic of the Spanish flu ended in around three years,"＝「スペイン風邪の流行は3年ほどで終わった」
"so I did not think that this corona pandemic would last so long,"＝「なのでこのコロナの流行がこれほど長引くとは思いませんでした」
"especially in this age of advanced medicine."＝「特にこの医療が進歩した時代に。」
ご参考に。
アメリカ合衆国 
