Two interventions dominate malaria vector control: long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). Both target the same insect at different moments in its feeding cycle, and both have decades of field evidence behind them. What the evidence does not support is the intuitive assumption that using both together, everywhere, produces a straightforward additive benefit — the cost-effectiveness picture is considerably more specific than that.
Two Tools, Two Points in the Mosquito’s Cycle
LLINs work as a physical and chemical barrier at the moment a mosquito attempts to feed on a sleeping person: the net blocks contact, and the insecticide woven into or coated on the netting kills or repels the insect that lands on it. IRS works later in the same cycle — insecticide applied to indoor walls and ceilings kills mosquitoes that rest indoors after taking a blood meal, before they can bite again or complete their reproductive cycle. Because the two interventions interrupt transmission at different points, they are often assumed to be complementary by default. Field evidence complicates that assumption considerably.
What WHO Guidance Actually Recommends
Current World Health Organization guidance does not recommend routine co-deployment of LLINs and indoor residual spraying across areas with ongoing malaria transmission. The stated priority is achieving high-quality implementation and strong coverage of one intervention rather than layering a second one to compensate for weak execution of the first — a poorly distributed net campaign is not fixed by adding spraying on top of it. Combining the two is instead reserved for specific circumstances: as part of an insecticide resistance management strategy, where a non-pyrethroid IRS product is paired with standard pyrethroid nets specifically to slow the development of resistance; historically, in epidemic response settings; and in areas experiencing residual transmission that persists despite good coverage of either tool alone, where the WHO frames Integrated Vector Management — combining LLINs, IRS, environmental management, and larval control — as the more effective long-term framework rather than simply stacking the two headline interventions.
The Cost Numbers Behind the Coverage Decision
The financial case for choosing one intervention over the other, or over combining them, is where the evidence gets concrete. In one Mozambique-based cost analysis, delivering LLINs to 80% coverage over three years cost roughly $2,200 per 1,000 people, compared with roughly $4,850 per 1,000 people for IRS at the same coverage level — more than double. The composition of those costs differs too: for LLINs, the net itself accounts for the large majority of the expense (around 88% of an estimated $10,641 annualized cost per 10,000 population), meaning procurement and distribution logistics dominate the budget line. For IRS, the insecticide product itself is typically the largest single cost component, representing roughly half of total campaign spending, which means IRS budgets are far more exposed to insecticide pricing and to the higher cost of non-pyrethroid formulations used where resistance is a concern.
On effectiveness, the two interventions have each shown strong independent results in different settings: LLINs have been associated with roughly a 50% reduction in malaria episodes and a 20% decrease in child mortality in endemic regions in pooled analyses, while consistent IRS application has been linked to reductions in malaria prevalence among children of up to 70% in some community-level studies. Neither figure transfers cleanly to the other intervention, and neither guarantees the same result when the two are combined.
When Combining the Two Is (and Isn’t) Worth It
This is the part of the evidence base that most often gets flattened into a slogan. A trial-based cost-effectiveness analysis conducted in Ethiopia found that combining LLINs and IRS was not more cost-effective than standard practice, even though a separate literature-based analysis in the same study suggested LLINs alone were likely a good investment on their own. A different analysis, from a high-transmission district in Mozambique, found the opposite in a narrower population: combining IRS and LLINs was highly cost-effective specifically for children under five, but not across all age groups — a gap the researchers attributed to the high cost of the organophosphate insecticide used in that IRS campaign, which raised the price of protecting the full population beyond what the added benefit justified.
The pattern that emerges across these studies is not “combine when possible” or “never combine” — it is that the value of combining LLINs and IRS depends on local transmission intensity, existing net usage rates, and, critically, the resistance profile of the local mosquito population. Where pyrethroid resistance is low and LLIN usage is already good, adding IRS rarely pays for itself. Where resistance is high and net usage is weak, a resistance-management pairing of non-pyrethroid IRS with LLINs can be justified. A procurement or program decision made without checking which of those conditions actually holds locally is a decision made on the slogan rather than the evidence.
Comparing the Two at a Glance
| Factor | LLINs | Indoor Residual Spraying |
|---|---|---|
| Point of interruption | Mosquito attempting to feed | Mosquito resting indoors after feeding |
| Approx. cost per 1,000 people (3-year, 80% coverage, Mozambique data) | ~$2,200 | ~$4,850 |
| Dominant cost driver | The net itself (~88% of cost) | Insecticide product (~51% of cost) |
| General cost-effectiveness position | Usually more cost-effective where resistance is low | More competitive where resistance is high and net usage is weak |
| WHO’s routine-use guidance | Prioritize high-quality coverage of one intervention | Reserve co-deployment for resistance management, epidemics, residual transmission |
Conclusion
The comparison between insecticide-treated bed nets and indoor residual spraying is not a contest with a universal winner. LLINs tend to be the more cost-effective default where resistance is manageable and distribution logistics work, while IRS earns its higher cost where resistance pressure or coverage gaps make it necessary. Combining the two is a targeted resistance-management and residual-transmission tool, not a default upgrade — the evidence is consistent that quality execution of one well-chosen intervention outperforms a poorly resourced attempt at both.