Mass deworming

What the evidence says

Rigorous studies disagree, so this entry is graded mixed. The full debate is in the Disagreement Register.

WHO estimates that 1.5 billion people, about 24 percent of the world, carry soil-transmitted worms, and it recommends periodic treatment of at-risk people without individual diagnosis. In 2021, more than 500 million children were treated.

The case for large benefits rests mainly on one program. Starting in 1998, a school deworming project in Busia, Kenya, phased treatment into 75 schools at random. Miguel and Kremer (2004) reported that absenteeism fell by about a quarter in treated schools, that untreated children nearby also benefited, and that test scores did not improve. A 2021 analysis reported about 14 percent higher consumption when pooling the 15- and 20-year follow-ups, and about 13 percent higher hourly earnings when pooling the 10-, 15- and 20-year follow-ups. These estimates, comparing people who had received two to three more years of treatment, were significant only at the 10 percent level; they were not both estimates from the 20-year follow-up alone.

The case against comes from systematic reviews. A 2019 Cochrane review of 51 trials concluded that mass deworming does not appear to improve height, hemoglobin, cognition, school performance or mortality. A 2017 review by Welch and colleagues found little to no effect on weight, height, cognition, attendance or mortality, and judged long-run evidence too weak to draw conclusions. The DEVTA trial of about 1 million preschool children in India found no significant effect on deaths or weight.

In 2015 independent researchers reanalyzed the Kenya data. After coding errors were corrected, they found little evidence that attendance rose in neighboring schools, while effects inside treated schools were largely unchanged. The original authors replied that the corrected data still support their findings.

GiveWell treats deworming as a bet. In 2016 it wrote that most of its staff thought deworming more likely than not to have very little or no impact, but that low costs, about $0.50 to $1 per person treated, keep its expected value high even after applying a 1 to 2 percent multiplier to the trial effects. Because the size of the benefit is the thing in dispute, no reliable per-dollar estimate of the outcome is published, and this entry has no Impact Calculator figure.

Where it sits on the depth scale

Deworming is prevention (depth 2). It clears infections and lowers transmission for a time, but treatment has to be repeated. WHO pairs it with hygiene education and better sanitation, the work that would cut transmission closer to its source.

Open questions

• Can large long-run gains exist when short-run health gains are hard to detect?
• GiveWell notes that infections in Busia were much heavier than in most places deworming happens today. Do the results carry over? See the replication rate.