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 time to kill two kinds of parasites: soil-transmitted helminths and the parasites that cause schistosomiasis.

In the case of deworming, the same children are being treated, on the same schedule, for similar diseases. In other cases, programs overlap enough to provide some opportunity for layering:

• Co-delivery of vitamin A supplementation and seasonal malaria chemoprevention. We’ve supported Malaria Consortium to leverage its existing door-to-door delivery of antimalarial medication to also provide vitamin A supplements.

Seasonal malaria chemoprevention is provided once a month for four or five months during the period of greatest malaria transmission. Vitamin A supplementation is typically provided twice per year. So the program is able to co-deliver one of the two vitamin A supplementation rounds—the one that coincides with the malaria season.

Not all layering happens during mass campaigns like the ones described above. We’ve also funded programs that leverage existing healthcare appointments by adding additional services:

• Syphilis screening and treatment in pregnancy. HIV testing is standard practice during antenatal visits in many countries. We’ve funded Evidence Action to support the governments of Zambia, Cameroon, and Liberia to switch from HIV tests to dual HIV/syphilis tests, providing a way to identify and treat syphilis as part of an existing health screening.

This program is cost-effective, in part, because it makes use of existing processes: pregnant people are going to the clinic anyway for their scheduled visits, and they’re already being screened.¹We’ve learned that building on existing healthcare touchpoints, particularly for vulnerable populations, is often cost-effective. That’s why many of the most promising layering opportunities we’ve found to date are for infants: infants have particularly high mortality rates concentrated in a short period, and routine childhood immunizations provide a number of touchpoints with the healthcare system during that time. As a result, right now our vaccines team is focused on layering additional interventions with vaccine delivery.

These are only a few of the programs we’re researching. We’ve also recently funded a program to promote breastfeeding by using an existing system that can send text messages to parents and caregivers (it was originally set up to promote childhood vaccination), and we’re investigating the example from the introduction: bundling delivery of chlorine and oral rehydration solution with childhood vaccinations.

Rather than taking a general position on whether layered interventions are cost-effective, we’re supporting them where they make sense and not where they don’t. We’ll continue to investigate possibilities as we seek the most cost-effective interventions of every kind.