Before revision:  Based on an extended SEIR model and continuous-time optimal control theory, in this paper the optimal non-pharmaceutical intervention strategy is presented for the case that a vaccine is never found  .
After revision:  Based on an extended SEIR model and continuous-time optimal control theory, in this paper the optimal non-pharmaceutical intervention strategy is presented for the case that a vaccine is never found and complete containment (eradication of the epidemic) is impossible .