Participants

We recruited participants in a similar way to previous investigations by local radio advertising and by writing to families who had participated in previous clinical trials and expressed a wish to participate in further research. Prospective participants were sent an information booklet and if, after reading, they wished to take part an appointment was made for an appropriate date to start the study.

Unlike other studies, only the youngest member of the family who was attending school was recruited to the study. Thus, the minimum age was 4 years and the maximum 16 years. Other members of the household were not included so they could act as potential sources of infestation for participants.

Everyone joining was conducted through a standardised consent/assent procedure and then assessed for presence of head lice using a plastic head louse detection comb (PDC, KSL Consulting, Denmark). This was mainly to provide information about the person's current risk status because everyone was treated to ensure all participants started free from infestation. Other household members who were infested at this time were offered treatment to reduce the risk of an immediate reinfestation pressure on the index member.

All participants gave baseline data on age, gender and hair characteristics as well as information on current medications and medical history. All treatments and assessments were conducted in participants’ homes. There was no payment for participation.

Eligibility and inclusion criteria were being of appropriate age, as described above; being at risk of reinfestation based on previous individual and family history and being willing to participate for the estimated 14 weeks of the study. Exclusion criteria were a history of allergy or sensitivity to components of the test product or placebo; of long-term scalp disorders, such as impetigo or psoriasis; pregnancy or breast feeding and participation in other clinical studies within 1 month prior to entry.

Ethics

A Clinical Trial Notification was also made to the Medicines and Healthcare Products Regulatory Agency in the UK, reference CI/2012/0032. Parents provided written consent for the participating child. Participants also provided written assent. Each participant's general practitioner was informed of their taking part.

The study was conducted in conformity with the principles of the Declaration of Helsinki and the International Conference on Harmonisation (ICH) Guidelines and European Standard for Good Clinical Practice (GCP).

Study medications

This was a randomised, double-blind, cross-over study of 1% 1,2-octanediol in a hair-conditioning base (Hedrin Protect & Go, Thornton & Ross Ltd, UK). It was supplied in 100 mL trigger spray high density polyethylene (HDPE) plastic bottles, used like a leave-in detangler conditioning spray, applied twice weekly to washed and towel-dried hair. More frequent applications were permitted during the 6-week period of use, for example, if the participant washed their hair more regularly. The placebo comparator was superficially identical and applied in the same way and at the same rate. Both required shaking before application and had a warning to avoid spraying onto the face to prevent eye irritation.

At enrolment and at cross-over between using the different treatment sprays, we provided treatment to all participants to eliminate any lice already present, even if none were detected. For this we used dimeticone 4% liquid gel (Hedrin Once liquid gel, Thornton & Ross, UK) applied for 15 min before washing, with a repeat treatment after 7 days, which was not strictly necessary due to the high level of efficacy exhibited by the product,9 but the second application was a requirement for approval of the study by the Medicines and Healthcare Products Regulatory Agency (MHRA) assessor. We used the same product to treat infestations that participants and household members acquired during the course of the study. Participants who were found to have contracted an infestation at any point were not withdrawn. Treatments were applied by investigators. Participants who had been infested continued to use their designated spray during the period between applications of dimeticone liquid gel because this therapeutic product is non-residual and thus conferred no protective effect between treatments.

At the beginning of the study an instruction sheet was supplied to the parent/carer(s) for use of the sprays. At weekly intervals an investigator visited each family to check the participant for lice using a detection comb, supply a new bottle of spray and return the used bottle to the study centre for weighing to determine the quantity used.

Definition of infestation

We expected that some lice would be found while using the preventive spray because it was possible that participants may have picked up lice at school during the afternoon prior to the visit. Therefore, no action was taken on first finding of lice unless there were five or more large lice (adult and third stage nymphs) or if there were any small nymphs (first and second stage nymphs) present. Either of these was evidence that an infestation had been present for some time. Young nymphs would only be present if eggs had been laid on the head and most reinfestation events start with fewer than five adult or third stage nymphal lice. If lice of any stage were discovered on two consecutive visits, this was considered primary evidence of an ongoing infestation. When infestation was confirmed it was treated using two applications a week apart of dimeticone 4% liquid gel, and any lice discovered were fixed into the case record using clear tape as confirmation.

At the end of each of the 12 weeks of follow-up, the assessor noted whether:

1. There were any live lice present.

2. Lice were found at previous assessment but no action was taken.

3. There were more than five lice.

4. Any stage 1 or 2 nymphs were present.

If an assessor found there were any live lice present (‘1’) and, if at the same time, any of ‘2’, ‘3’ or ‘4’ also applied, this was considered to be an active infestation. The lice were collected and fixed into the case record book, and the participant treated to eliminate infestation. The numbers of each development stage, and the total numbers of lice were recorded after examination in the laboratory.

Objectives

The study objective was to demonstrate that with regular use, 1% octanediol spray could protect against head lice establishing an infestation by killing any lice that crawled onto the treated hair. Unlike a repellent, we recognised that lice would not be inhibited from crawling onto the head but that the product should, if it was applied correctly, be effective in limiting the risk of an infestation becoming established for people using it.

Outcomes

The primary outcome measure was the time to first infestation, identified using systematic detection combing over the whole head. Secondary endpoints were whether infestations occurred at any time while using the product and the safety of the spray in use.

Sample size

This study was designed to detect the superiority of 1% octanediol product compared with a placebo. The study was of an unusual type for clinical investigations because, unlike most clinical investigations, the participants in this study did not already have a treatable condition. The aim to prevent a treatable condition was also unlike other ‘preventive’ studies, for example, vaccine trials, in that those are normally long-term population studies engaging large numbers of participants with a quite small potential for detectable failure overall.

We proposed a cross-over design because it allowed smaller numbers of participants to be involved, also allowing each participant to act as his/her own control. Developing the design was difficult because the risk factors for each individual were unknown, so randomisation alone may not wholly address any disparity in infestation risk due to social and family circumstances, especially in a relatively small study cohort. Consequently, self-controlling for each individual was an attractive option to avoid any skew resulting from these unknown factors.

Primary analysis, based on time-to-onset of first infestation, was considered a more powerful method of detecting differences between 1% octanediol and placebo than a simpler approach based on whether or not a participant got an infestation.

Sample size calculations by the statistical consultant (see online supplementary file) were based on 10 000 simulations of cross-over studies using a range of defined study sizes, setting the power to detect a difference significant at the 95% CI, and then estimating the minimum sample size to obtain 80% or 90% power. For risk of infestation, we looked at the experience of participants in previous studies of between three and five instances of reinfestation per year, estimated to be equivalent to a rate per person per week of about 6–10%. From this we expected a reduction of risk between 60% and 70% when using the active spray. Consequently, we selected a sample size of 68 participants based on assumed weekly infestation rates of 6% for placebo and 2% for octanediol, based on the estimated sample size for 80% power of 64 plus allowance for dropout. These were equivalent to weekly rates of survival from infestation of 94% for placebo and 98% for octanediol, or 6-week survival rates of 69% and 88.6%. This sample size gave expectations of 19.8 possible infestations for placebo and 7.8 for octanediol over the course of the study.

Randomisation: allocation concealment

The randomised treatment allocation code was generated using the free online randomisation service at http://www.randomization.com/, seed number 26 438 created on 10 October 2012. Treatment allocation was made in eight balanced blocks of 10 treatments, with one spare block randomised in case replacements were required.

The treatment allocations bore the anonymous identification of the product to be used and instructions for application. The product identification/instruction sheets were sealed in opaque sequentially numbered envelopes with the participant number taken from the randomisation schedule. This study operated in a cross-over design with each participant acting as their own control so all participants used both preparations during the course of the study. The product codes were not broken until after the completion of data collection, entry of data into the study database and database lock.

Statistical analysis

Analyses testing for differences between the treatments accounted for the cross-over design and were based on within-participant differences between effectiveness of 1% octanediol compared with placebo during the respective 6-week treatment periods. Primary data management and analyses were performed by PN Lee Statistics and Computing Ltd in collaboration with the investigators. Binary data were analysed using the McNemar test and counts and ranked data using the Wilcoxon signed-rank test for paired data. We analysed participants overall and separately in each randomisation arm, according to which treatment they received first.

For the primary outcome, the time to the first confirmed infestation, we used a seven-point ranking to score the participants:

1=Infestation first confirmed at the first follow-up assessment

2=Infestation first occurred at the second follow-up assessment

....

6=Infestation first occurred at the sixth follow-up assessment

7=No infestation confirmed in the six assessments

Other endpoint analyses included whether infestation occurred at any time, how many new infestations occurred during each 6-week treatment period and the number and types of adverse events.

For the primary outcome, we used Kaplan-Meier curves to illustrate the time pattern of survival of the participants from infestation either when using 1% octanediol or when using placebo. We did not test differences between the treatments for significance using the log-rank test because the two curves were non-independent being based on the same participants.

We performed analyses on the intention-to-treat (ITT) and per-protocol (PP) groups. Prior to the start, we anticipated some dropouts, mostly during the second 6-week period of treatment. In order to address this problem, if it arose, we planned analyses to allow for dropouts by making an assumption that this would be due to infestation. Thus for analysis of dropouts we assumed that an infestation had occurred in the first week that a follow-up was not possible. If this were to happen in the first treatment period, so there were no data for the second period, we made the assumption that the same response would have occurred in both 6-week periods. However, based on previous experience in this community, we also anticipated that dropout, were it to occur, would arise at a very low rate that would not require censoring or other specific measures to address the issue in the analyses.

We also analysed baseline characteristics to compare participants according to which product they were randomised to receive first. These data were compared using Fisher's exact test for binary data and the Mann-Whitney U test for counts and ranked variables.