CD4 and sickle cell disease: Of the 21 travellers who received the YFV, individuals were 60 or over (n = 4); without co-morbidities (n = 2), and with co-morbidities such as coeliac or diabetes (n = 2), individuals between 6 months and 2 years old without underlying health conditions (n = 2), or with sickle cell disease (n = 1), aged 18–59 living with virologically supressed HIV CD4 > 200 (n = 7), sickle cell disease (n = 3), diabetes (n = 2) or on low dose antimetabolites (n = 2).