1 Introduction

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, is a respiratory illness outbreak unlike the world had seen in recent history, and led to over 760 million cases and 6.9 million deaths between 2019 and 2023 [1, 2]. Public health teams worked tirelessly across the world testing a variety of solutions to reduce the effects of the ongoing threat. Outbreak response methods traditionally focus on case identification, halting transmission, surveillance, and isolation of cases; techniques that have been implemented and built upon from early century disease [3].

Contact tracing is an important tool for outbreak management as it assists in breaking chains of transmission to slow or stop the spread of a virus [4]. Traditional methods of contact tracing require manual efforts by teams of public health officials who trace the close contacts of identified index cases presenting with the disease of interest and provide support with the quarantine of contacts [4, 5]. Although traditional methods of emergency pandemic mitigation techniques were beneficial in fighting the evolving COVID-19 disease, many countries utilized information systems and technology, including smartphones, to take advantage of a world in which technology is an everyday part of life. In 2019, 65% of the global population were smartphone users, a statistic that aided public health response teams as digital contact tracing (DCT) applications and technology were incorporated into outbreak management [3].

DCT applications provide the unique opportunity to identify close contacts of positive COVID-19 cases without the work required to be performed by manual contact tracing teams which is demanding in time, effort, and relies heavily on the quantity and quality of information available [5]. Countries who elected to adopt DCT technology typically did so in the form of smartphone applications, websites, and wearable technology that provided the user with the status of infections in a specific location they had visited or sending exposure notifications (ENs) to close contacts of cases. Many DCT applications were built upon location-based technology including Bluetooth, GPS, QR Code, or a combination of these technologies [5, 6].

International stakeholders play an important role in the development of digital technologies for contract tracing. Go.Data is a digital tool used to combat the Ebola epidemic and it was developed by WHO in 2014 [7]. WHO and PAHO also had developed a multidisciplinary information hub to enable exchange of information on COVID-19 contact tracing protocol [8]. Other institutions, like CDC, conducted research focusing on requirements for digital contract tracing application to provide guidance for technical implementation of DCT [9]. This highlights the collaborative efforts aimed at sharing information and best practices for contact tracing protocols, underscoring the importance of global cooperation in addressing public health emergencies.

DCT has previously been used in other outbreaks including the use of the DCT platform for Polio in Nigeria which was implemented in 2017 using Geographic Information Systems and the use of Ebola Contact Tracing app in Sierra Leone as well as SORMAS (Surveillance, Outbreak Response Management and Analysis System) in Nigeria during the 2014–2016 Ebola epidemic [10,11,12,13]. Other technology related contact tracing effort that has been recorded includes the use of public surveillance cameras, the closed-circuit television (CCTV), in China and South Korea during the MERS outbreak in 2015 [14, 15].

DCT technologies uptake play a role in outbreak management. It was estimated that 60% uptake rates in the population have substantial effect on reducing the spread of disease and even lower numbers of 15% would still reduce infection and death rates [16,17,18,19]. To ensure that DCT technology'zz's uptake in the population reaches the desired numbers and is effective in outbreak management, we need to have a holistic understanding of the characteristics of DCT technology that is most suitable to be implemented for an emergency response outbreak.

This systematic review will provide analysis of the characteristics of the implemented DCT technologies during COVID-19 pandemic in order to better understand the potential use of technology for future outbreak response. An overview will describe the DCT features such as the origin, DCT system ownership, technology and platform type, as well as mandate status of the DCT technologies.

2 Methodology

2.1 Search strategy

The processes conducted for this review were based on the PRISMA guidelines. A systematic search was carried out using Embase (1974–2023), MEDLINE (1946–2023), and PubMed (-2023) databases on May 20th, 2023. Both Embase and MEDLINE were searched via Ovid, whereas PubMed was searched via NIH ncbi.nlm.nih. Two search strategies were created to be used for the different databases. The search terms used for the Embase and MEDLINE databases through OVID are “covid-19 or covid19 or sars-cov-2 or coronavirus or severe acute respiratory syndrome coronavirus”, “contact tracing or contact investigation or transmission tracing or contact with infect* or contact follow-up or case detection”, “mobile phone* or smartphone* or cell* phone or social media or phone app* or personal device or digital device), 1 and 2 and 3, “limit 4 to english language”, and “limit 5 to yr = ”2019-Current””. The search terms used for PubMed are “covid-19 or covid19 or sars-cov-2 or coronavirus or severe acute respiratory syndrome coronavirus” AND “contact tracing or contact investigation or transmission tracing or (contact adj3 infect*) or contact follow-up or case detection” AND “mobile phone* or smartphone* or cell* phone or social media or phone app* or personal device or digital device”.

Studies eligible for review were study that fit the following criteria; (i) digital technology used for public health contact tracing during COVID-19 pandemic; (ii) published between December 2019-May 2023; (iii) peer-reviewed published research including cross-sectional studies, cohort studies, case–control studies, case series, case reports, meta-analysis; (iv) primary or secondary outcomes report digital technology in the public health sector; (v) written in English language. Studies that are predominantly designed as dissertation, thesis, conference abstract, randomized control trials, background review, any type of expert opinion, letter to editor, commentary, pilot study, and modelling study; non contact tracing outcome; outcome not based on digital technology performance; written in non-english language were excluded from review process.

2.2 Selection process

The selection process ensured that duplication papers were removed before entering the reviewing stage. Multiple reviewers were used in the study selection phase to reduce error and possible selection bias. The first and second reviewers independently completed title and abstract screenings as well as the full text screening using EndNote20, Excel, and Google Sheets. Studies were screened in the title and abstract phase with inclusion and exclusion criteria, and if given information was not enough to decide, the study was forwarded to the full text screening for a more in-depth review. Studies meeting the inclusion criteria, reported data on the technology name, and one or more other categories during the full-text screening, were included in the final review. Studies identified for inclusion by both the first and second reviewer during screening stages were selected for the data extraction phase, and studies identified for exclusion by both reviewers were removed. Any discrepancies between the first and second reviewers were examined by a third reviewer, who used the final decision to reach a consensus between two reviewers.

2.3 Data collection and analysis

Data collection was conducted by the first reviewer from the sixty-one included studies which passed the screening phases. A template was developed using Microsoft Excel and Google Sheets to outline information needed to be extracted from these studies.

Each study was examined for data fitting each of the outcome categories, namely: i) name of the digital contact tracing technology; ii) DCT implementation location which consists of continent, country, and region; iii) year of DCT release; iv) DCT system ownership, categorized as government, private, and government-private collaboration; v) targeted user demographic, for health workers or for public; vi) purpose of technology development, categorized as developed for COVID-19 and repurposed; vii) type of technology used for location tracking such as Bluetooth, GPS, or QR codes; viii) type of platform for the DCT technology such as applications, websites, wearable devices; xi) mandate status, whether the app was government mandated or voluntary used, and other characteristics such as unique features, number of downloads, number of contacts identified, detection rate, and number of cases prevented. Information was not reported in all categories for each DCT technology and missing data was designated as not available.

3 Results

3.1 Study selection

The search of all three databases produced a total of 1,254 results, which were imported into EndNote20. From these results, 434 were duplicates and removed by both EndNote20 and manual assessment, leaving 820 to be screened for eligibility. The first phase of screening, by title and abstract, resulted in 683 records being excluded for either meeting exclusion criteria or not meeting inclusion requirements. The second phase of screening assessed the full text of each study and an additional 76 studies were excluded for one of six reasons: study design, results did not provide outcomes for technology and performance, not contact tracing, not digital contact tracing, and study not in English. An additional study was excluded because the full text could not be accessed. The resulting number of studies passing eligibility screenings and were available for extraction was 61 [5, 6, 16, 18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75]. Figure 1 shows a PRISMA [83] flow chart of the selection process during each stage.

Fig. 1
figure 1

PRISMA flowchart with results for screening process

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3.2 Digital contact tracing technology characteristics

The studies reported 147 DCT technology apps, websites, and wearable technology. Of the 147 included, 11 were from Africa (7.5%), 54 from Asia (36.7%), 35 from Europe (23.8%), 36 from North America (24.5%), 3 from Australia/Oceania (2.0%), 5 from South America (3.4%), and 3 did not have an originating location identified (2.0%). Figure 2 depicts the coverage of digital contact tracing technology on a global scale by identifying the number of DCT technologies adopted per country.

Fig. 2
figure 2

Number of DCT technologies used per country

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This map demonstrates that many countries adopted only one form of DCT technology, while several had two or three. India, China, and the United States of America have the highest populations in the world [14], and were the only countries to report more than four applications or other technology available for use. Data was collected on characteristics of technology reported on; the summary of these findings is reported in Table 1. A full report of characteristics for each DCT technology is included in the data analysis table (Online Resource 1).

Table 1 Summary of the characteristics assessed of DCT technology
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Out of 147 DCT technologies reviewed, 67 DCTs were released in 2020, showing an effort to catch up with the peak of COVID-19 pandemic. Majority of the DCT technologies (65;75.6%) were developed and owned by the government, some are owned by private sectors (20; 23.3%) and only a miniscule amount is a collaborative effort of government and private sectors (1;1.2%). As a DCT technology, the purpose of the development matches the current outbreak, which is for COVID-19 tracing (53; 96.4%). The preferable type of technology used in development of DCT technology is bluetooth (70; 70%), followed by GPS (30; 30%) and QR codes (22; 22%). Application becomes the chosen platform for DCT technology (130; 90.9%), followed by applet (9; 6.3%) and wearable devices (2; 1.4%). There are only 2 DCT technologies that reached 100,000,000 downloads/uses (3.1%). Most of the DCTs had a number of uses ranging from 1,000,000 - 9,999,999 downloads (18; 27.7%) and 10,000 - 99,999 downloads (13; 20%). Considering the number of downloads, it is consistent with the result that only 6 of them are mandated to be used (6;27.3%) and the majority are voluntarily used by the population (14;63.6%).

Studies reported the number of downloads and uses of DCT technologies as either a number or percentage of the study population. Percentage and population size were not reported for all studies at the time data was collected, therefore it is difficult to determine the exact number of downloads for numerous apps. The DCT which had the highest number of downloads was reported to be the Health Code apps from China which received 1.28 billion downloads country-wide between all provincial versions. As an individual DCT technology, the Aarogya Setu app launched by the Government of India received 216 million downloads. The number of downloads or uses of DCT organized by continent is shown in Fig. 3.

Fig. 3
figure 3

Number of Downloads or Uses by Continent

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As seen in Fig. 3, four applications with download counts in Africa, Australia/Oceania, and South America reported results between 1,000,000 – 99,999,999 downloads. Europe reported downloads for 13 applications throughout countries to be between 10,000 – 99,999,999. Both Asia and North America received a much wider range of download/use of technology with Asia reporting ranges from 1,000 all the way up to 100,000,000 + for 16 technologies, and North America within the range of 1 to 99,999,999 for 29 applications.

In Figs. 4A & B, features of technologies were compared by continent to identify differences between regions and preferred technology used for DCT. Overall, 75.6% of the technology reported government DCT system ownership. Africa, Asia, and North America reported technology launched by both private and government bodies (n = 69, 80.2%), while Australia/Oceania, Europe, and South America only reported government affiliated applications (n = 14, 16.3%), and three (n = 3, 3.5%) were unidentified in location. North America reported the highest parentage of privately affiliated companies at 34.3% (n = 12) of North American DCT technologies. Three technologies were reported as privately affiliated with unknown originating locations. Technology types were broken into 4 categories: Bluetooth, GPS, QR code, and other. In general, in general, the most adopted technology type was Bluetooth, followed by GPS and QR code. Applications that reported more than one type of technology used were included in all appropriate categories. Multiple technology types were reported in all continents excluding South America, which had results of technology type for only one of the five applications included in the review, and therefore reported only the use of Bluetooth. Europe and North America heavily favoured Bluetooth technology with fewer DCT applications utilizing GPS, QR Code, or other technology. Africa, Asia, and Australia/Oceania used a variety of Bluetooth, GPS, QR Codes, and other technology types.

Fig. 4
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A DCT system ownership by continent, B Technology type by continent, C Technology platform by continent, D Area coverage by continent

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Technology platforms were examined across the continents and findings are reported in Fig. 4C. Overall, applications were the most used DCT technology platform. North America reported a small percentage of SSO (single sign-on) identification methods used for DCT, the majority done through smartphone applications. Asia reported slightly less percentages of DCT technology using apps, but use of Applets accessed through already existing applications as well as wearable devices and websites. Africa, Australia/Oceania, Europe, and South America reported the sole use of smartphone applications for digital contact tracing. Three applications were identified without corresponding locations. The area of coverage per technology was also collected and reported upon in Fig. 4D. Most technologies in all continents, excluding North America, reported coverage country-wide and a few were applied across multiple countries. Of the applications launched for each European country, many reported the ability to be used in surrounding countries. North America reported very different results as most technology in the United States of America provided coverage by state or county, and Canada received provincial coverage from two of the three applications reviewed.

4 Discussion

This review identified 147 digital contact tracing technology implemented in 83 countries across the world. There was a global implementation of technology that reached every continent, but fewer DCT technologies were available in Africa and South America than any other continents. Countries without reported technology correlate with nations of less developed status, and those with more numerous available DCT technologies were of those most highly populated [76, 77]. Applications and other technology used within Asia reported the highest number of downloads and uses, followed by Europe, North America, South America, Australia/Oceania, and Africa.

Although China’s Health Code application system was the only technology to receive uptake of over 80% of the population, other technologies reported over the recommended 60% threshold, and most DCT technology received more than the recommended 15% of population minimum. Mandate status may correlate with the increased number of downloads and uses, as those which were deemed mandatory for use did not report results with low number of uses. Privacy and ethical issues have been raised as a possible deterrent to uptake of DCT technology within the public [78].

DCT technology was generally affiliated with government rather than private institutions. Although governments may have partnered with private companies to develop the technology, those which were approved or owned by government departments were considered affiliated with a country’s governmental body.

Not all applications and wearable devices used more than one form of technology type, but many applications used Bluetooth the primary proximity indicator within DCT and incorporated the use of GPS, QR Codes, and other technologies to assist with accuracy. Bluetooth technology was typically used to trace contacts by identifying other Bluetooth compatible phones through signals ranging 6 feet or less for more than 15 min [35, 79]. Signal transmissions were commonly described as “handshakes” between Bluetooth compatible devices [46]. QR Codes were multifaceted in its applications, finding different uses between a variety of apps. Select applications used QR Codes in facilities, venues, and borders, requiring users to scan a code with their app, logging visits by individuals with their key personal information [80]. Other applications generated QR Codes to provide personalized health status, vaccination status, and travel passes [45].

Smartphone applications were reported as the most used platform of technology for DCT world-wide and by a large margin. Wearable technology was implemented in countries which had already adopted smartphone applications to include those who did not have access to their application. Singapore provided wearable devices, called tokens, which complimented the TraceTogether application to address technical and privacy concerns hindering the uptake of the DCT technology [81]. This helps to reduce inequalities in populations and close gaps caused by lack of access to smartphones which are compatible with these technologies [24].

Lastly, the majority of DCT technologies were implemented countrywide, allowing for more contacts to be traced with movement of cases and health populations. Many European countries offered applications, which could still be used in neighbouring countries allowing for better digital contact tracing efforts to be conducted. Liu reported this feature in apps including, but not limited to, HOIA, #OstaniZdrav, Corona-Warn-App, CoronaMelder, and Koronavilkku [82]. However, this was not the case within the United States of America which based the coverage of their applications on state lines rather than as a country overall. If applications do not correspond with those in other states, travellers pose additional risks to spreading disease with the ability to digitally trace.

4.1 Strength and limitations

This study provides a comprehensive perspective on the implementation of digital contact tracing technologies used during covid-19. By reviewing 147 DCT technologies across 83 countries, this study also covers the analysis of prominent characteristics of DCT technology that influence its adoption, providing valuable insights to enhance understanding towards effective contact tracing efforts. There are limitations identified within this review. Only studies which were reported in the English language were included leaving the possibility of quality data being excluded, especially with many of the technologies covering countries which do not use English as an official language. Additionally, many studies were limited and did not provide information for several of the characteristics of interest. Although the selection process was designed to reduce bias, quality assessments for individual studies were not conducted, leaving room for other biases to be overlooked, and it is noted that publication bias may exist within the studies available. It is important to review the differences and similarities summarized in this review between DCT technologies to understand how different technological, and political factors relating to mandate status, may play a role in their performance. Outbreaks will continue to threaten public health so by utilizing the information gathered within this systematic review, DCT developers and government institutions can prepare and identify changes that may be made to existing technology prior to when it is needed next. Further research should be conducted to determine the effectiveness of DCT in contact tracing methods and outbreak management as this review was unable to collect the required data to make analyses.

5 Conclusions

The COVID-19 pandemic posed a serious global public health threat that opened opportunities for advanced technology to be implemented in outbreak management strategies. This systematic review focus on landscape of digital contact tracing (DCT) technologies deployed during the COVID-19 pandemic. Findings showed a global disparity in DCT availability, with fewer implementations observed in Africa and South America compared to other continents, indicating a correlation with the level of economic development and population density. Notably, government affiliation predominated in the development and deployment of DCT technologies, with mandates potentially influencing uptake rates.

Technologically, Bluetooth emerged as a primary proximity indicator within DCT, often augmented by GPS, QR codes, and other tools to enhance accuracy. Smartphone applications emerged as the most utilized platform for DCT worldwide, although efforts to bridge technological disparities were evident through the introduction of wearable devices in regions lacking smartphone access, as seen in Singapore. Moving forward, further research is warranted to assess the effectiveness of DCT in contact tracing and outbreak management. This study provides a comprehensive analysis of digital contact tracing technology during the COVID-19 pandemic, highlighting global implementation across 83 countries. By leveraging the findings of this study, policymakers, DCT developers, and public health officials can proactively prepare and refine existing technologies to address future public health crises, thereby contributing to the global effort to combat infectious diseases.