This includes, for example, those who have received organ transplants, and those who are living with cystic fibrosis and some types of cancers of the blood or bone marrow.
Mae hyn yn cynnwys, er enghraifft, y rhai sydd wedi derbyn trawsblaniadau organau, a’r rhai sy’n byw gyda ffibrosis systig neu ganserau penodol, fel canser y gwaed neu fêr yr esgyrn.