Before revision:  iii ) the key role played by the quarantine in the prevention of the spread of Covid-19 in terms of an interacting \emph{ parameter, which emulates the contact between infected and non-infected people.
After revision:  iiiiii ) the key role played by the quarantine in the prevention of the spread of Covid-19 in terms of an interacting \emph{ parameter, which emulates the contact between infected and non-infected people.