This will be a Phase Ib open-label trial of CAVATAK™ (CVA21) in combination with Pembrolizumab for the treatment of patients with advanced NSCLC. The dose of Pembrolizumab will be fixed at 200mg. Three cohorts (dose levels) of intravenously-delivered CVA21 will be explored, using a standard 3+3 patient dose escalation design. The starting dose of CVA21 will be one log below the 1 x 10^9 TCID50 dose found to be safe when CVA21 was given alone in an ongoing Phase I study (NCT02043665).