Intestinal worms infect hundreds of millions of children in places without sanitation. The treatment is a pill that costs pennies, given once or twice a year at school. Evidence Action’s Deworm the World program puts the all-in cost at about 44 cents per child per year, including delivery.
So the question is not whether it is cheap. It is whether it does anything.
The health evidence: mostly null
Cochrane’s 2019 review pooled dozens of trials of mass deworming and found little or no effect on weight, hemoglobin, cognition or school attendance in the year or two after treatment. That is the strongest kind of evidence we have for short-term outcomes, and it says the average child in a mass program gets no measurable health benefit.
Part of the reason is that mass programs treat everyone, including the majority who have light or no infections. Trials that treated only heavily infected children did show gains.
The income evidence: large, from one place
In 1998 to 2001, a program in western Kenya randomized the order in which schools got deworming, so some children received two to three extra years of treatment. Researchers have followed those children ever since.
The 20-year results, published in PNAS in 2021, found that the treated group had 14% higher consumption and 13% higher hourly earnings as adults. They were 9% more likely to live in cities and worked 9% more hours in non-farm jobs. Tracking rates were high (84%), which matters for a follow-up this long.
If those numbers are right, deworming is one of the best investments in human capital ever measured: a few dollars per child, returned many times over in adult earnings.
Why the fight
In 2015 a reanalysis of the original Kenya study found coding errors and questioned some of its results, setting off what economists called the “worm wars.” The reanalysis confirmed the effect on school attendance but disputed the mechanism. The 20-year follow-up came after that and used a pre-registered plan, which is why it is taken more seriously than the original.
The remaining objection is simpler: it is one study, in one region, at one time. Worm loads in western Kenya in 1998 were high. Whether a program in a lower-burden area in 2025 produces anything like a 13% earnings gain is unknown, and there is no second 20-year trial to check against.
How GiveWell handles it
GiveWell funded deworming as a top charity for a decade, then in 2022 moved it into its broader grant-making rather than its headline list. Its reasoning is instructive: it takes the Kenya income result seriously but applies a heavy discount for the chance that it does not generalize, and for the lower worm burdens where programs now operate. Even with those discounts, the program’s near-zero cost keeps it competitive; GiveWell continues to grant to deworming programs at a lower estimated cost-effectiveness than its top charities.
How to hold both
- The health case is weak. Do not give to deworming because it makes children healthier this year; the trials say it mostly does not, on average.
- The income case is strong but thin. One well-run, long-run study shows a large effect. It might be real and general, or real and local.
- The price changes the calculation. At 44 cents a child, deworming does not need to work well to be worth it. It needs to work a little, some of the time.
That is a reasonable bet for a donor who accepts uncertainty. It is not the “proven” intervention some fundraising copy calls it, and it is not the debunked one its critics call it either.
Method notes
Cost per child is Evidence Action’s figure. Trial results are as reported in the cited papers. GiveWell’s position is our summary of its published program page and grant write-ups.
Sources
- Hamory, Miguel, Walker, Kremer, Baird: Twenty-year economic impacts of deworming (PNAS, April 2021)
- Evidence Action: The 44-cent treatment that dramatically changed lives in Kenya
- Cochrane review: Deworming drugs for soil-transmitted intestinal worms in children (2019)
- GiveWell: Combination deworming (mass drug administration targeting both schistosomiasis and soil-transmitted helminths)
- World Bank Development Impact blog: Worm wars, a review of the reanalysis of Miguel and Kremer's deworming study
Figures come from the cited sources as of the dates shown. Cost-effectiveness estimates change as programs and evidence change; check the source before giving. We are not financial advisers and receive nothing from any charity named here.