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