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The COVID-19 pandemic represented an exceptional threat to human health worldwide. In that context, the collective adoption of protective behaviors (eg, hand hygiene, physical distancing, mask wearing) became an essential measure to prevent the transmission of the virus, especially during the pandemic outbreak when there was no vaccine available [,]. Consequently, the development of effective behavior change techniques aiming at supporting the uptake and adoption of such behaviors emerged as a public health priority []. However, during the course of the outbreak, there was limited or no contextualized knowledge about how to effectively promote protective behaviors in the general population [,].
During the early stages of the pandemic, we addressed the call for applying principles of behavior change to reduce the transmission of COVID-19 [,] and devised a Multiphase Optimization Strategy (MOST) [] to develop, optimize, and evaluate a smartphone-based behavior change technique to promote hand hygiene during the COVID-19 pandemic [,]. We opted for a digital intervention to avoid in-person interaction, which is crucial for preventing virus transmission. Additionally, digital interventions can be tailored to individual needs and have the potential to reach a large number of users. The MOST framework was deemed highly suitable for rigorously contextualizing the intervention to the pandemic through optimization. We targeted correct hand hygiene (ie, hand washing or sanitizing) at key times because it represents an effective strategy in preventing the transmission of respiratory illnesses, including COVID-19 [,], and was therefore included in public health guidelines [].
Following the MOST framework [], the project included 3 phases: preparation, optimization, and evaluation. In the preparation phase [], we developed a set of intervention modules and defined the optimization criteria. The development of intervention modules was guided by the Theoretical Domain Framework []. First, we identified behavioral determinants of hand hygiene through a literature review, and through 2 focus group discussions with the target population []. Then, we mapped behavior change techniques [] to the behavioral determinants. Specifically, three intervention modules were developed targeting (1) motivation, (2) habit formation, and (3) social influence. The identified modules were paired and sequenced, resulting in 9 different intervention conditions. Each intervention condition was hosted in the developed smartphone app Soapp. During the subsequent optimization phase, we devised a mixed method double-blind parallel randomized trial between March 2021 and August 2021, which lasted 34 days. The trial aimed at identifying the intervention condition that met our optimization criteria: superiority regarding change in hand hygiene pre-post intervention, and regarding user satisfaction, usability, and engagement []. The results of the trial showed that hand hygiene improved while using the Soapp app. However, no superiority was found for any of the versions of the app. The qualitative results indicated that participants preferred the motivation and habit formation modules over the social influence ones. A further emerging theme pointed out the need for better distribution of intervention content over time. Thus, in the optimized version of the Soapp app, named Soapp+, we selected the motivation and habit formation modules and delivered them in a parallel fashion rather than sequentially.
Soapp+ comprises behavior change techniques aimed at targeting key motivational and volitional behavioral determinants including intention, attitudes toward hand hygiene, risk perception, outcome expectancies, self-efficacy, action planning, coping planning, action control, and habit. These determinants represent fundamental psychological constructs in prominent behavior change theories [], addressing both reflective and automatic processes. Reflective processes are deliberate and conscious, involving setting intended goals, planning, and monitoring progress [,]. In contrast, automatic processes, such as habit, operate unconsciously, enabling individuals to perform behaviors without deliberate thought once learned through repetition. These automatic processes help sustain behavior change by reducing the cognitive load associated with decision-making and promoting consistency in behavior performance [,].
AimsThe current paper focuses on the evaluation phase of the MOST in which we tested the efficacy of Soapp+ (intervention group) by comparison to an active control group in a randomized controlled trial (RCT). As presented in the study protocol [], we tested the following preregistered hypotheses
H1: The intervention group shows a greater increase in correct hand hygiene behavior at key times at the postmeasure (H1a) and at the 6-month follow-up (H1b) than the control group.H2: The intervention group shows a significant increase in the targeted behavioral determinants compared with the control group at the postintervention measure (H2a) and at the 6-month follow-up (H2b). Specifically, the target behavioral determinants were intention, attitude, risk perception, outcome expectancies, self-efficacy, action planning, coping planning, action control, and habit strength.As the secondary aim, we examined between-group differences in flu-like infection symptoms and occurrences of COVID-19 (secondary outcomes). The corresponding results are reported in .
The study design for the evaluation phase was a double-blind RCT (1:1 ratio) comparing the optimized app Soapp+ with an active control group using a simplified version of the app that focused on providing information on hand hygiene. The trial was carried out from March 9, 2022, to April 18, 2023, a few days before the World Health Organization declared the end of the global health emergency [].
ParticipantsThe target population was German-speaking adults from the Swiss population interested in using an app to improve hand hygiene. Inclusion criteria were (1) being at least 18 years of age, (2) owning a smartphone with mobile access to the internet, (3) being proficient in the German language, and (4) having signed an informed consent form to participate in the study. Additionally, smartphone proficiency was an implicit eligibility criterion, as participants needed to perform various tasks on their smartphones prior to the study’s commencement (eg, downloading the app and registering). As per protocol [], the initial target sample size was 205 participants. The sample size was calculated a priori for an independent sample 1-tailed t test (β=.80; α=.05; Cohen d=0.35). The target sample size was raised to 245 participants to account for a 20% dropout rate.
MeasuresPrimary OutcomeThe frequency of correct hand hygiene at key times was assessed using ecological momentary assessment with an electronic diary embedded in the study apps (ie, Soapp+ and active control apps). This approach was used to avoid retrospective bias in reporting hand hygiene []. On study days 2, 8, 16, 24, and 32, participants were prompted 5 times per day to indicate whether each of the 13 key times to perform hand hygiene defined by the Swiss Federal Office of Public Health occurred (eg, arriving home, after using the toilet; Table S2 in ). For each key situation that occurred, participants were asked how often they correctly washed or disinfected their hands. The 5-point response scale ranged from 0=never to 4=always. For each diary day, the hand hygiene score was calculated by (1) computing the mean frequency of correct hand hygiene across all indicated key times in a single diary and then (2) averaging the hand hygiene frequency across the five diaries.
Secondary OutcomesTargeted Behavioral DeterminantsThe following behavioral determinants were assessed at T1, T2, and the follow-up. These variables were selected based on the results of the optimization phase and represented the determinants targeted by the Soapp+ intervention.
Behavioral IntentionOne self-reported item was adapted from a study by Allan, Sniehotta, and Johnston [] asked “To what extent do you intend to correctly perform your hand hygiene behavior at key times?” The response options ranged from 1=not at all to 6=very strongly.
Action PlanningThis was assessed using the mean score of 3 self-reported items [], such as “I have made a detailed plan about when I am going to wash and disinfect my hands.” The response options ranged from 1=not at all to 6=very strongly. The scale was reliable (Cronbach α=0.90 at T1).
Coping PlanningThis was assessed using the mean score of 4 self-reported items, such as “I have made a detailed plan about how I can perform correct hand hygiene behavior at key times when soap and water are not available.” The response options ranged from 1=not at all to 6=very strongly (Cronbach α=0.84 at T1).
Habit StrengthThis was assessed with the “Self-Report Behavioural Automaticity Index” [] with 4 items, such as “Correct hand hygiene behavior at key times is something that I do automatically.” The response options ranged from 1=strongly disagree to 7=strongly agree (Cronbach α=0.93 at T1).
Action ControlThis was assessed using the mean score of 3 self-reported items, such as “During the last two weeks I have constantly monitored myself whether I washed or disinfected my hands according to my plans.” The response options were adapted from a study by Sniehotta, Scholz, and Schwarzer [] and ranged from 1=not at all to 6=very strongly (Cronbach α=0.91 at T1).
Self-EfficacyThis was assessed using the mean score of 8 self-reported items, such as “How confident are you that you correctly wash/disinfect your hands at key times, when you are on the way.” The response options ranged from 1=not at all to 6=very strongly. Self-efficacy items were adapted from a study by Schwarzer [] (Cronbach α=0.91 at T1).
AttitudeThis was assessed using the mean score of 6 self-reported bipolar items [] introduced by the wording “Correct hand hygiene behavior at key times is…” and followed by pairs of bipolar adjectives (eg, useless vs useful; Cronbach α=0.81 at T1).
Outcome ExpectanciesThis was assessed using the mean score of 8 self-reported items [], such as “If I engage in correct hand hygiene, then then the risk drops that I contract pathogens.” The response options ranged from 1=this doesn’t apply at all to 6=this applies very precisely (Cronbach α=0.72 at T1).
Risk PerceptionThis was assessed using the mean score of 5 self-reported items [], such as “to what extent do you think that you could contract corona, if you don't engage in correct hand hygiene in key situations?” The response options ranged from 1=this doesn’t apply at all to 6=this applies very precisely (Cronbach α=0.90 at T1).
Flu-Like Infection Symptoms and Occurrences of COVID-19Participants reported on these secondary outcomes by answering the items “In the last two weeks, did you have flu-like infection symptoms?” and “In the last two weeks, did you have a positive corona test?”
Finally, social desirability was assessed at T1 using the German Short Scale for the two-dimensional measurement of social desirability [].
ProcedureParticipants were recruited via social media (eg, Facebook, Instagram), mailing lists, and leaflets with the support of a market research company. We aimed at recruiting a broad range of people from the German-speaking adult Swiss general population in terms of sex, age, and socioeconomic status. The content of the recruitment materials targeted individuals interested in forming new hand-hygiene habits and learning how to correctly wash or disinfect their hands to counter the spread of the virus. Interested people were redirected to the study page on RedCap (Vanderbilt University) [] where they could go through the study information, fill out an eligibility and consent survey, and sign the e-consent form. Afterward, participants received a registration code via email and were guided to download fthe Soapp+ app from the Apple App Store or Google Play Store and register it. The day after the registration, participants received the baseline questionnaire (T1) and were then randomized to an intervention or active control group. A simple randomization was implemented in Qualtrics, which preserved the allocation concealment. Additionally, the researchers involved in the study were blinded to intervention assignment because the participant identifier was pseudoanonymized before randomization (Figure S1 in ). During the intervention phase, participants completed the hand hygiene diary on days 2, 8, 16, 24, and 32. At the end of the intervention phase (day 34), participants received a second questionnaire (T2). At 6 months (182 days) after T1, participants received the last follow-up questionnaire. Questionnaires and diaries were integrated into Qualtrics services using Soapp+’s application programming interface, and the participants’ data were stored on Qualtrics. The recruitment for the evaluation trial began on March 9, 2022, and ended on October 20, 2022. Follow-up data were collected between September 5, 2022, and April 18, 2023.
InterventionThe content of the intervention was delivered to participants via their personal smartphone through the study app Soapp+. The intervention group received the optimized Soapp+ app while the active control group was exposed to a simplified version of Soapp+. shows the content delivered. The exact wording of the intervention messages and the intervention timeline can be found in Table S3 and Figure S2 in .
Table 1. Intervention content overview.Condition and moduleTDFa domainBehavioral determinantBehavior change techniqueIntervention and active control groupsaTDF: theoretical domains framework [].
Active Control GroupThe active control group received information on how to perform correct hand hygiene (both in written and video formats), the key times to perform hand hygiene, and a statement of the goal of the app (basic module). This content was accessible throughout the intervention phase. Based on the results of the optimization phase, we also added a list of fun facts that were delivered to the participants via push notifications throughout the intervention phase. Finally, quizzes on hand hygiene aimed at ensuring that the active control group interacted with the app throughout the intervention phase.
Intervention GroupLike the control group, the intervention group received the basic module. They additionally received the content targeting motivation and habit. Based on the results of the optimization phase (ie, request for better-distributed content or tasks), this content was delivered in parallel instead of sequentially as done during the optimization trial.
Data AnalysisHandling of Missing DataTo handle missing data, the intention-to-treat (ITT) principle was adopted []. According to the ITT principle, all the randomized participants are included in the analysis, regardless of what happens after randomization (eg, noncompliance, protocol deviations). Therefore, the ITT approach avoids overoptimistic estimates of the efficacy of an intervention resulting from the removal of noncompliers. Missing data and dropouts were addressed using the last observation carried forward approach. The last observation carried forward approach represents a parsimonious yet effective method to maintain appropriate type 1 error protection []. This approach has been used in previous behavior change research [], including the optimization trial of the Soapp app []. This methodological choice was reinforced by the results of a secondary analysis conducted on the optimization trial []. The analysis revealed a noteworthy intraclass correlation of 0.60 in hand hygiene, signifying that a substantial portion of the observed behavior can be attributed to individual differences between persons. This implies that the behavior tends to be more consistent or less variable within individuals over time. Finally, we also conducted a set of sensitivity analyses without missing value imputation; results are available in .
Hypothesis TestingIn order to test the hypotheses, the assessment points considered for hand hygiene behavior were the first diary on day 2 (T1), the diary day on day 32 (T2, postintervention), and the last diary day on day 181 (follow-up). The difference in hand hygiene between T2 and T1 and follow-up and T1 was compared between the intervention and the active control groups using the nonparametric Wilcoxon rank sum test. A positive value in hand hygiene difference over assessments indicated an increase in hand hygiene. Likewise, positive values in the targeted behavioral determinants over assessments indicated an increasing trend. The Wilcoxon rank sum test was chosen over the preregistered 1-tailed Student t test due to the nonnormal distribution of the data, as assessed by the Shapiro-Wilk normality test, representing a deviation from the registered protocol. Results of the analysis with the Student t test are available in Table S4 in .
Analytical SoftwareThe Soapp+ app was developed by the study authors using technological resources provided by the Faculty of Human Sciences at the University of Bern, with support from an internal software engineer. The statistical software R (version 4.3.0; R Foundation for Statistical Computing) was used to process the data and run the analyses. The data and R code used for the main analyses are available on the Open Science Framework repository platform [].
Ethical ConsiderationsThe trial received ethical approval from the Swiss Ethics Committee of the canton of Bern (ID 2021-00164). The reporting of the trial is in line with the CONSORT (Consolidated Standards of Reporting Trials) guidelines () []. Informed consent was obtained from all participants, who were also informed of their right to opt out at any stage of the study. To protect privacy and confidentiality, data were deidentified through pseudonymization. As compensation, participants had the opportunity to win one of three iPhone 12s as an incentive for their participation. Additionally, after completing T2, participants were offered a small gift (ie, a bar of hand soap and a thank you card) in order to prevent dropout, which was sent to their homes.
We stopped the trial 13 months after the start of the study due to the end of the project timeline. A total of 193 participants were recruited and randomized into one of the 2 experimental conditions. Among these, 12 participants did not complete any of the 6 hand hygiene diary days, while the other 35 participants did not fill out the first diary at T1. Further, 3 participants completed the first diary but did not encounter any of the key situations to perform hand hygiene during that day. These participants (n=50) were excluded from the analysis because the main outcome (ie, hand hygiene) at T1 was missing. Out of the 193 participants who were randomized, 146 (75.6%) filled out the hand hygiene diary at T1, 91 of these (47.2% of the randomized participants) completed the hand hygiene diary at T2, and 109 (56.5% of the randomized participants) completed the hand hygiene diary at the 6-month follow-up. shows the participants’ flow through randomization, T1 diary assessment, T3 diary assessment, and follow-up for each intervention group.
Figure 1. Participant recruitment flow. Baseline CharacteristicsSociodemographics and hand hygiene behavior at baseline are reported in (see Table S1 in for a comparison with the sociodemographic characteristics of the Swiss population). The figures refer to the 146 participants who completed the first diary at T1. Participants’ mean age was 41 (SD 17) years, 69.2% (n=101) were women, 69.9% (n=102) had high school qualifications, 45.9% (n=67) were employed, and 24% (n=35) were living alone. Descriptive statistics in hand hygiene behavior (mean 3.05, SD 0.78; median 3.25, IQR 2.63-3.60; skewness=–1.24) suggested that hand hygiene behavior was already high at baseline and characterized by a moderate left-tailed distribution. No significant differences in social desirability were found between the intervention and the control group (M1=4.72; M2=4.79; F1,189=0.44; P=.51).
Table 2. Sociodemographic characteristics at baseline (T1; N=146).VariableOverallExperimental groupP valueaaWelch 2-sided t test or Fisher exact test.
bSwiss francs. When the study started in March 2022, the average conversion rate between Swiss francs and US dollars was 0.93, meaning that US $1 was equal to 0.93 Swiss francs
Dropout AnalysisDropout analysis was performed to investigate baseline differences between participants who reached the end of the intervention phase and those who dropped out at any point during the intervention. We analyzed all 193 randomized participants and the ones who did not complete the T2 panel assessment were categorized as dropouts (n=120). The results suggested that participants who dropped out were significantly younger than retainers (M1=39; M2=46; F1, 191=7.06; P=.009). Dropouts and retainers did not differ regarding sex (male vs female; χ21=1.72; P=.19), hand hygiene (F1,190=0.12; P=.73), intention to increase hand hygiene behavior (F1,191=2.36; P=.13), or intervention group allocation (χ21=2.04; P=.15).
Hand Hygiene BehaviorHand hygiene behavior at T1, T2, and follow-up is summarized in . The whole sample exhibited a nonsignificant increase in hand hygiene behavior from T1 (median 3.25, IQR 2.63-3.60) to T2 (median 3.42, IQR 2.62-3.88; W=8904; P=.06, r=0.11). This was followed by a significant decrease between T2 and the 6-month follow-up (median 3.25, IQR 2.50-3.65; W=11,655; P=.04; r=0.12). When looking at changes in hand hygiene within each experimental group, the data shows that hand hygiene between T1 and T2 remained almost stable for the active control group (W=2050; P=.77; r=0.03), while the intervention group showed a significant increase in hand hygiene after the intervention phase (W=2386; P=.02; r=0.19). At follow-up, both the active control group (W=2313; P=.35; r=0.08) and the intervention group (W=2924; P=.68; r=0.03) exhibited a nonsignificant decrease in hand hygiene compared to T1.
The between-group differences in hand hygiene behavior between T1 and T2 and T1 and follow-up are reported in . Nonparametric analysis with the Wilcoxon rank sum test showed that the change in hand hygiene behavior between T1 and T2 (W=2034; P<.04; effect size r=0.17) and between T1 and follow-up (W=2005; P<.03; effect size r=0.18) significantly differed between the groups, indicating better hand hygiene in the intervention group.
Table 3. Hand hygiene and targeted behavioral determinants median values at each assessment point and Shapiro-Wilk normality test.Variable and assessment pointParticipants, nOverall, median (IQR)Active control group, median (IQR)Intervention group, median (IQR)Shapiro-Wilk normality test
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