Increasing impact by combining programs

The idea has obvious intuitive appeal: If you’re already sending community healthcare workers door-to-door in (say) remote parts of Sierra Leone to deliver routine childhood vaccines, why not have those healthcare workers deliver chlorine for disinfecting drinking water, or oral rehydration solution for treating dehydration from diarrhea?
After all, if you’re already spending money on the fixed costs of delivery, why not provide other programs at the same time? You’d be able to amortize the costs across multiple goods and offer additional benefits to the community. (If you’re getting groceries delivered, it’s more efficient to have one driver deliver your eggs and milk and vegetables all together than to have separate drivers going round delivering each one separately.)
GiveWell is very interested in these “layered interventions,” and we are excited to support them wherever they cross our cost-effectiveness threshold. But we’ve discovered it’s harder than you might think to find ways to combine programs effectively.
If layered interventions are so intuitively appealing, what makes them more challenging in practice? In short: for a layered intervention to work, two different commodities need to be relevant to the same people, at the same time, on the same schedule, with compatible delivery logistics.
Often, layering just doesn’t work. For example, when distributed via a mass campaign, malaria nets are usually distributed to all households in a community every three years. It probably wouldn’t make much sense to use those campaigns as opportunities for childhood vaccination because most routine early childhood vaccinations are scheduled to be administered by trained health care workers during the first 18 months of life. The timing and processes don’t line up.
It might seem like deworming campaigns, in which children are given medication to kill parasitic worms, might layer with seasonal malaria chemoprevention, in which children are given medication to prevent malaria. But there are challenges there too: deworming happens once or twice a year in areas with high worm burden; in contrast, seasonal malaria chemoprevention happens once a month for several months during the malaria season in areas where malaria is highly seasonal. The timing and locations don’t necessarily line up.
However, sometimes layering works well and is quite cost-effective:

Combination deworming. We’ve supported deworming programs (for example, here and here) that provide two different kinds of medicine at the same

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