We think of design as an ingredient when it is used in combination with other approaches across the program cycle. In such programs design might be brought in to de-risk the solution development process by ensuring that early prototypes are put into the hands of users. Or a thorough design research process may enable you to reach populations whom you might not have reached before, or in a deeper and more immersive way, thus enabling a stronger focus on equity.
Design can de-risk a process through its iterative nature of taking learning, making it tangible, putting it into people's hands, and continuously refining from feedback. It helps us to get out of a situation where we invest in the design of a solution, devoid of user feedback, and pilot it, only to find out after the fact that it wasn't what people needed or wanted. Including a design partner in your investment will ensure that you successfully put into place an iterative learning and prototyping process.
This will also have an effect on the partners you choose and how they work together. In these projects it is common to default to contracting to an implementing partner, given their deeper familiarity with a geography or common issues in public health. However, when the contracting mechanism assumes the implementing partner will sub contract to a design firm, it is not always the case that they have the requisite experience with design to know when and how design should be integrated to be most effective.