This study makes use of two independent nationwide panel studies from Switzerland to assess adherence to multiple preventive measures once SARS-CoV-2 vaccines became available to wider populations in early 2021. We observed a dichotomy of participant groups who did not use the digital proximity tracing app during the study period and did not get vaccinated, and another group who used the app during the study period and got vaccinated. Consistent with our hypothesis, we found that participants had a higher chance of using the app or taking additional preventive measures leading up to vaccine uptake. Similarly, participants had a lower risk of uninstalling the app following vaccine uptake.
Our findings reveal that people who use the app and adhere to preventive measures had a higher aHR for vaccine uptake. This is aligned with findings by Ferretti et al. highlighting the potential effectiveness of digital contact tracing app use and conventional preventive measures for transmission mitigation until vaccines became available to wider populations.2 In particular, we observed that individuals who were possibly more concerned about the pandemic, such as those who were at higher risk of contracting the virus or were more vulnerable to severe illness, got vaccinated earlier.27 Furthermore, our results are aligned with observed high adherence to preventive measures during the initial phases of the SARS-CoV-2 pandemic in Switzerland.26–28 They also suggest a possible perceived benefit to combine multiple preventive measures to reduce onward viral transmission leading up to vaccine uptake. In Switzerland, this was observed in a study conducted during the first wave of the pandemic, which found that adherence to multiple preventive measures contributed the most to reduce viral transmission, with only 4% of the transmission reduction resulting from natural immunity.29 This is also aligned with previous studies underscoring the lack of effectiveness of individual preventive measures to reduce onward viral transmission alone30–33 and the benefits of combining them to enhance each measure.34–37
In our study, we assessed a period of the pandemic when there were increasing concerns over the vaccine’s ability to reduce the spread of the highly transmissible alpha (widely known as B.1.1.7) and delta (widely known as B.1.617.2) variants,38,39 and there was only limited evidence on the ability of vaccines to reduce onward viral transmission.40 At that time, in Switzerland, public health guidance kept recommending to adhere to preventive measures despite widespread vaccine rollout.41 As such, our assessment likely reflects a time during the pandemic when vaccination was perceived as more beneficial for individual protection from serious illness, rather than population-level protection. In our study, we observed a decreased aHR in app uninstalling after vaccine uptake for both assessed cohorts. This finding suggests a perceived benefit for people to extend their individual protection from vaccines to public level protection through continued use of the SwissCovid app.
Contrary to our trade-off hypothesis, we did not find a significant trend of reduced app use and preventive measure adherence after getting vaccinated. However, high counts of app uninstalling were observed, with approximately 7% (pooled study population) of self-reported SwissCovid app users at baseline uninstalling the app following vaccine uptake during the study period. Observed app uninstalling might be due to a lack of perceived benefits from the app or due to experienced difficulties in using the app.8,42 App uninstalling among vaccinated participants, particularly, can also be explained by ongoing misconceptions during the alpha and delta variant waves that vaccination could prevent all SARS-CoV-2 infections, making app use seem unnecessary.43
Nevertheless, our failure time analyses depicted a dichotomy of participants who did not use the SwissCovid app and did not get vaccinated (16% of pooled study population) versus participants who used the app and got vaccinated (40% of pooled study population) during the study period. These findings suggest a possible lower acceptance or increased mistrust of public health measures in reducing viral transmission over time, regardless of increasing incidence during the alpha and delta variant waves.44,45 Furthermore, they can point towards a general lack of public acceptance of the continued use of novel health technologies in public health. This underscores the need to gain a deeper understanding of the perceived utility of digital proximity tracing apps in public health to increase their use in future pandemic contexts.4 As such, public health authorities can benefit from adopting communication strategies that focus more on enabling trust and, as a result, higher compliance to preventive measures and using digital proximity tracing apps.11,46,47
Our study presents some limitations. First, in both the CI-DFU and CSM there may have been self-selection during enrollment that may have led to study participants with higher digital or health literacy, and higher socioeconomic status than the general population. Here, self-selection could have also been in the form of people taking part in the surveys as an additional measure to contribute to the public pandemic response. Second, both panel surveys were based on self-reports and possibly subject to common measurement biases due to socially desirable responding, which may have led to an overestimation of our study’s outcomes of interest. Third, although substantial efforts were presented by the CI-DFU to streamline data collection across various geographical regions in Switzerland, local heterogeneity in the implementation of the questionnaires was not avoidable at times, which may have contributed to disparate results. Fourth, the proportional hazards assumption was violated in the multivariable analyses with the CSM cohorts. However, sensitivity analyses with point-process Poisson regressions suggested the aHR for vaccine uptake increased over time and the aHR for SwissCovid app uninstalling decreased over time, which both confirm the results obtained with the CI-DFU cohort. Fifth, reporting of SwissCovid app uninstalling outcomes may not be suggestive of an active choice to not use the app as a preventive measure anymore but, e.g., participants buying a new phone and not reinstalling the app. Similarly, not reporting SwissCovid app uninstalling outcomes does not necessarily mean that the app was in use. Lastly, our approach to analyze the data with the last observation carried forward (LOCF) method for our exposures of interest may have introduced bias in estimating our study’s effects of interest, even though only a small amount of data in both study cohorts was affected by conditional study participation.48