Why Are Randomised Controlled Trials Important?


The importance of randomisation
Randomised Controlled Trials (RCT) are considered the gold standard in study design. Randomisation is important because if done properly it will minimise selection and other types of bias. Vector control tools will often work at the community level (for example, long lasting insecticidal nets can help protect everyone in the area, not just those sleeping under the nets), and in these cases it is important that randomisation takes place at the appropriate group or ‘cluster’ level, whether this is household, village, or district.Studies involving interventions at the individual level should also be randomised, but they can have problems of contamination; if different individuals in a community are given different interventions, such as insect repellents or repellent clothing, any sharing of these products with other individuals would interfere with the study results. It is therefore often preferable to allocate the intervention to individuals in a cluster, then randomise the clusters.It should be noted that while RCTs are rated as high-quality evidence by the GRADE methodology, studies can be up- or downgraded based on several factors: RCTs can be downgraded if there is risk of bias, inconsistency, or imprecision for example, and a non-RCT could be upgraded if a large effect size is observed. Nonetheless, non-randomised control trials are not recommended as selection bias is likely to be high and there are commonly no pre-intervention data to assess the comparability of test groups. Observational studies, such as case-control or cross-sectional studies, provide relatively weak evidence to support the effect of vector control interventions and rank below non-RCTs (Figure 2).
Weaknesses in vector control studies
A common problem in vector control studies is with implementation and adherence to the intervention. If an intervention, such as the use of repellents or nets within the home fails to have an effect, it is difficult to know whether this is because it did not work as expected or because it was not used as recommended. Therefore, it is important to ensure high coverage and monitor user compliance during the trial. Some studies use education and communication aimed at behaviour change to encourage user compliance2 or may employ random spot checks, and supervision3.Implementation and adherence to the intervention | Study designs should focus on quality control, high coverage and user compliance. Without these it will not be possible to determine the reasons behind any apparent lack of efficacy. |
Choice and measurement of outcome measures | Entomological outcomes can demonstrate proof of concept but epidemiological outcomes are needed to demonstrate the efficacy of an intervention in protecting people. |
Avoiding performance bias | Where possible, blinding of trial participants, health-care workers and others involved in a trial should be practiced to avoid introducing biases. |
Selection of sites for entomological monitoring | Sites should be selected in a consistent way across intervention and control arms of the study; random selection avoids sampling bias. |
Contamination or spillover effects | Clusters, such as villages, schools or households, should be well separated by a buffer zone to ensure there is no common boundary between the intervention and the control groups |
Need for sample size calculations | Sample size calculations should be performed before conducting a study to confirm that the study has the power to show an effect. |
Duration of the follow-up period | Follow-up periods need to be sufficiently long, with repeat measures, to gain an accurate assessment of the intervention effect. For RCTs this is likely to be at least one transmission season. |
In conclusion
Assessments of vector-borne interventions that are not rigorous and evidence-based can waste a considerable amount of money, time, and energy. Given the lack of vector control studies with epidemiological outcomes, and the development of new vector control tools for the control of Aedes mosquitoes, it is crucial that future studies are designed to provide high-quality evidence. Have you taken part in a randomised or non-randomised trial? Are you currently working on a trial? How do you think your study would rank in the GRADE methodology?Preventing the Zika Virus: Understanding and Controlling the Aedes Mosquito

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