There are remarkably few studies on the impact of ITN on infection prevalence among African communities living under conditions of low transmission 
and none we can identify undertaken in the semi-arid areas of Africa where transmission is maintained by An. arabiensis
. After a limited period of national net distribution in Somalia we have shown that among the varied communities of South Central Somalia net use remains low with only 12.4% of residents reporting using a net (). Among children aged below 5 years 12.4% () were reported using a net compared to only 8.7% reported during a cluster sample survey in the same regions 8–10 months earlier 
. Among non-net users P. falciparum
infection prevalence was overall 17.0% ranging from 7.9% among the riverine communities to 25.3% among the pastoralists. We cannot explain the paradoxical observation that communities closer to rivers had lower infection prevalence and would require a more comprehensive vector biting and larval entomological survey to explore this further. Similar findings, however, have been reported in The Gambia where communities living near mosquito breeding sites had proportionately lower parasite rates 
. Nevertheless the ranges of reported prevalence support the view that transmission intensity in this area of Somalia is between hypo- and mesoendemic. The overall age, sex and livelihood adjusted protective effect of nets among these communities was 54% ranging from 60% among pastoralist communities to 48% among communities located along the rivers (). The largest effects were seen among pastoralist and agro-pastoralist children aged 5–14 years. We suspect that most of these nets were treated with insecticide and the majority of those treated were LLIN (see methods
), however, the specifics of net treatment were not recorded.
The protective effect of nets on infection prevalence among the sampled populations in South Central Somalia was consistent across age-groups (). Given the relatively higher prevalence of infection through older childhood and into adulthood it is important to recognize the need to provide ITN to all members of a community and not focus only on young children in areas of low transmission. This resonates with recent calls for high coverage among all community members across the range of transmission settings 
where it is also recognized that individuals older that five years contribute to transmission.
Several studies in stable, hyper- holoendemic areas of Africa have examined the impact of nets and ITN on parasite prevalence. The meta-analysis of randomized controlled trials (RCT) suggests that the median protective effect of ITN on infection prevalence in children aged less than 15 years is only 13% 
. Including an examination of non-RCT studies it is clear that the ranges of reported protection vary widely largely a result of different studies examining different age groups and using different designs. This makes comparisons difficult and presents several interpretation problems. First, investigations that have been part of randomized controlled trials, where efforts are made to maximize coverage of net or ITN use among communities neighboring “control” communities may have under-estimated the effects upon infection prevalence 
due to a shadowed, wide-area protection provided by the intervention areas across control areas 
. Second, the combined active clinical detection surveillance and treatment among the same children examined for the prevalence of infection will bias the results away from significance as more control children will receive effective treatment during the surveillance period if nets protect against clinical events 
. These combined effects might explain why several investigations using single cross-sectional studies of infection prevalence against the reported use of nets and ITN under routine operational delivery conditions have tended to provide higher estimates of protection (51–63%) than those described during an RCT design 
. Finally, studies that have examined reductions in parasite prevalence due to nets or ITN among older children in high transmission areas will have been unable to identify the true impact on the incidence of infection through a simple cross-sectional estimate of prevalence 
. The duration of sub-patent or un-treated infections in an individual host may be many months making the distinction between old and new infections difficult due to saturation of multiple infections in high transmission areas. Conversely, in very young, immunologically semi-naive infants at Kilifi, on the Kenyan coast, comparisons of those sleeping under ITN with community randomized control infants showed that infection rates were reduced by 50% among those using ITN 
. Similarly an operational effectiveness study of ITN protection against infection prevalence in children aged less than 24 months in Tanzania showed a 62% protection against infection prevalence 
Our findings among a wider age group living in an area with a low rate of parasite exposure might therefore be expected to correspond to levels of protection described among young infants living in much higher transmission settings in Kenya 
and Tanzania 
. Our findings are also consistent with observations made during non-RCT investigations of net/ITN use in The Gambia 
and the unstable, highland areas of Kenya 
. The review by Lengeler (2004) might have under-estimated the impact on the prevalence of infection consequent upon ITN use. We would argue that the incidence of new infections might be reduced by over 50% in all transmission settings but measurement, through prevalence surveys, is critically dependent upon the selected age range in different areas of differing intensities of malaria transmission.
Halving the risks of P. falciparum infection among all age groups through the use of nets in an area of low transmission intensity has not been previously described among the semi-arid areas of Africa. These findings appear at first glance to support the wide-scale use of ITN in these areas. However, coverage remains poor and many more people would need to be reached to achieve a significant population-attributable impact. This then leaves an issue hard to resolve with the current data. Where infection prevalence is very low, such as the communities in the South Central region of Somalia, when does it become cost-ineffective to deliver ITN to the entire population? Covering 1.6 million people with an ITN to reduce infection risks from 17% to 7% might be seen as an expensive way to tackle the problem of malaria in this region. The answers to this dilemma would require a better understanding of the health risks consequent upon infection to compute a cost-per-disability-adjusted-life-year averted and a comparison with other approaches to malaria infection reduction in semi-arid, semi-nomadic and conflict areas of Somalia.
A particular challenge in Somalia, due to the large pastoral community, is maintaining a consistent survey sampling frame and in some cases there are difficulties in locating nomadic pastoralist clusters as seen in this study. To minimize the effect of this, a large sample size was therefore selected. In addition, our estimates of protective effectiveness on infection prevalence might have increased if it were possible to separate out ITN from untreated nets and estimate the impact of ITN alone. We present these estimates of impact under routine delivery conditions where we had no opportunity for strict randomization criteria nor, because of the opportunistic nature of the surveys, have we been able to adjust for the many other potential confounders of infection and net use for example socio-economic status or treatment seeking behaviours. However, the results were internally consistent between groups sharing similar livelihoods and economic activities adding confidence to the observed protective effects.