RNA encoding an immunogen is delivered to a large mammal at a dose of between 2µg and 100µg. Thus the invention provides a method of raising an immune response in a large mammal, comprising administering to the mammal a dose of between 2µg and 100µg of immunogen-encoding RNA. Similarly, RNA encoding an immunogen can be delivered to a large mammal at a dose of 3ng/kg to 150ng/kg. The delivered RNA can elicit an immune response in the large mammal.