TNF and viral infectious disease: These travellers were exempt due to history of thymectomy (n = 1), taking a TNF inhibitor (n = 1), natalizumab (n = 2), HCST within 24 months (n = 1), taking ustekinumab (n = 1 where timing of yellow fever was discussed for three months after they stop taking this monoclonal agent), recent high dose steroid courses (n = 1, YFV was discussed after disease had stabilized), cladribine (n = 1), and high dose mycophenolate (n = 1).