Is it permissible to miss performing Fajr prayer in the mosque due to contracting the coronavirus, and what is the expiation for that, and is this considered a covenant or a vow?

Your oath to pray in the mosque for life is an oath to an act of obedience, which it is recommended to fulfill. If you break it, expiation becomes obligatory upon you, and your conscience is cleared by performing it. However, if you return and swear or make a covenant with Allah, this is another oath, which it is recommended to fulfill. If you break it, another expiation becomes obligatory upon you. If you cannot feed the poor, then fast. Your illness does not nullify the expiation unless you had made an exception for the state of illness. Scholars differ regarding a covenant, and the view that it is an oath is easier for you; so if you break it, only expiation becomes obligatory upon you. Congregational prayer in the mosque is recommended, not obligatory.