Primary audiences are those that receive content directly. Secondary audiences are those that receive the content indirectly because they hear about it or are affected by it.
In general, you will be concentrating on the needs of your primary audiences. But it is a good idea to also consider how your secondary audiences may receive your content. For example, think about the level of knowledge and any potential biases the audience may have, and develop content to take these into account:
You are developing a fact sheet to tell health practitioners (primary audience) about vaccination. The health practitioners will use this information to both guide their practice and answer any questions from their patients (secondary audience).
In drafting content, think about what the health practitioner must know to provide the correct treatment. You can also think about the sort of questions that patients might ask about vaccine safety and efficacy, and include clear explanations that the practitioner can use.