Containment, the key strategy in quickly halting an epidemic, requires rapid identification and quarantine of the infected individuals, determination of whom they have had close contact with in the previous days and weeks, and decontamination of locations the infected individual has visited. Achieving containment demands accurate and timely collection of the infected individual's location and contact history. Traditionally, this process is labor intensive, susceptible to memory errors, and fraught with privacy concerns. With the recent almost ubiquitous availability of smart-phones, many people carry a tool which can be utilized to quickly identify an infected individual's contacts during an epidemic, such as the current 2019 novel Coronavirus (COVID- 19) crisis. Unfortunately, the very same first-generation contacttracing tools can also be -and have been -used to expand mass surveillance, limit individual freedoms and expose the most private details about individuals.
We seek to outline the different technological approaches to mobile-phone based contact-tracing to date and elaborate on the opportunities and the risks that these technologies pose to individuals and societies. We describe advanced security enhancing approaches that can mitigate these risks and describe trade-offs one must make when developing and deploying any mass contact-tracing technology. Finally, we express our belief that citizen-centric, privacyfirst solutions that are open source, secure, and decentralized (such as MIT Private Kit: Safe Paths) represent the nextgeneration of tools for disease containment in an epidemic or a pandemic. With this paper, our aim is to continue to grow the conversation regarding contact-tracing for epidemic and pandemic containment and discuss opportunities to advance this space. We invite feedback and discussion.
Infectious diseases spread in an exponential fashion. Containment is an effective means to slow the spread, allowing health care systems the capacity to treat those infected. However, 'lock down' like containment can also disrupt the productivity of the population, distort the markets (limiting transportation and exchange of goods), and introduce fear and social isolation for those that are not yet infected or that have recovered from an infection. Finally, and most importantly, contact tracing can be quickly deployed at the first warnings of an outbreak, but continues to be effective when disease resurgence concerns exist. Thus, following an initial epidemic peak, contacttracing can be an effective means to enable disease decline and avoid multiple peak periods and disease resurgence.
Lessons from China have suggested the utility of understanding GPS localization of intersections between known infected individuals and others in stemming infection progression. This is specifically related to the R0 (R naught) that determines how contagious an infectious disease is. R0 is a description of the average number of people who will catch a disease from one contagious person. Ideally, a lower number will optimize reduction of disease spread, which will facilitate time to develop a vaccine or for the disease to die out. Three factors that define R0 are the infectious period (which is generally fixed for a given disease), the contact rate (i.e., how many people come in contact with a contagious person), and the mode of transmission (which is similarly fixed for a given disease). Thus, for a given disease, the most adjustable factor is the contact rate.
One key issue with contact rate is how to optimally allow individuals and societies to limit the contact rate. Contact amongst uninfected individuals will not facilitate disease spread. Thus, ideally a society and/or an individual is principally concerned with understanding the contacts an infected individual has had. Understanding if paths have been crossed between an infected individual and any number of other individuals will allow for identifying those who have been exposed (and maybe should be tested resulting in appropriate resource allocation or may isolate themselves in the absence of available testing). Thus, at a societal level, this may limit the economic and public impact.
With an application that allows for users to understand potential exposure to an infected individual, and appropriate action of the exposed individuals, it may be possible to reduce the contact rate by more rapidly identifying cases/exposures which will remove them from the contact chain. For example, if we assume uptake of an application amongst x% of a population, and assuming that portion of the population responds to known exposure by self-quarantining or pursuing texting to confirm lack of infection, the R0 will decrease in turn by a multiple of that percentage based on the degree of mixing in the population. The reason for the multiple decrease is R0 partially depends on the population size and density and the exact number of people an individual may come in contact with after exposure which varies amongst individuals. Furthermore, with an increasing number "x" in terms of user base, there will be an exponential decrease in R0 (e.g., for 100% use and appropriate action, R0 would be expected to fall <1 due to maximal reduction of contact rate). Thus, for example, a 10% uptake will have downstream impacts on individuals that person may have come in contact by more rapid exposure/contact identification. This may eventually disrupt the contact rate with may significantly reduce the R0 more than is accounted for by the 10%.
This ultimate effect of R0 with a 10% use and appropriate response to data will hopefully disrupt ongoing chains of transmission, thus effecting the mortality rate and eventually impacting the contact rate and infection curve. However, high enough utilization could reduce contact rate to such a degree as to make the overall R0 < 1 which would ideally lead to dying off of the infection entirely.
Almost half of the world's population carries a device capable of GPS tracking. With this capability, location trails-timestamped logs of an individual's location-can be created. By comparing a user's location trails with those from diagnosed carriers of infectious disease, one can identify users who have been in close proximity to the diagnosed carrier and enable contact-tracing. As the COVID-19 outbreak spreads, governments and private actors have developed and deployed various technologies to inform citizens of possible exposure to a pathogen. In the following, we give a brief overview over these technologies.
We take this opportunity to define several critical terms used throughout this paper.
• Users are individuals who have not been diagnosed with an infectious disease who seek to use a contact-tracing tool to better understand their exposure history and risk for disease.
• Diagnosed carriers then, refers to individuals who have had a confirmatory diagnostic test and are known to have an infectious disease. Of note, in the setting of an epidemic in which some infected individuals have mild or no symptoms, a subset of users will in fact be unidentified carriers. An inherent limitation in all containment strategies is the society's ability to identify and confirm disease
• Location trails refer to the time-stamped list of GPS locations of a device, and presumably therefore, the owner of the device.
• Finally, we broadly speak of the government as the entity which makes location data public and informs those individuals who were likely in close contact with a diagnosed carrier, acknowledging that this responsibility is carried out by a different central actor in every continent, country or local region.
• Local businesses refer to any private establishment such as shops, restaurants or fitness clubs as well as community institutions like libraries and museums.
Broadcasting refers to any method, supported by technology, by which governments publicly share locations that diagnosed carriers have visited within the time frame of contagion. Governments broadcast these locations through several methods. For example, Singapore updates a map with detailed information about each COVID-19 case. South
Korea sends text messages containing personal information about diagnosed carriers to inform citizens. In the US, Nebraska and Iowa published information of where diagnosed carriers have been through media outlets and government websites. Broadcasting methods can be an easy and fast way for a government to quickly make public this information without the need for any data from other citizens. It requires citizens to access the information provided and evaluate whether they may have come in contact with a diagnosed carrier of a pathogen themselves. However, broadcasting methods risk exposing diagnosed carriers' identities and require exposing the locations with which the diagnosed carrier interacted, making these places, and the businesses occupying them, susceptible to boycott, harassment, and other punitive measures.
Selective broadcasting releases information about locations that diagnosed carriers have visited to a select group, rather than the general public. For example, information might be selectively broadcast to people within a single region of a country. Selective broadcasting requires collection of information, such as a phone number or current location, from users in order to define the selected groups. Often, a user must sign up and subscribe to the service, e.g., via a downloaded app.
Selective broadcasting operates under one of two modes: (i) The broadcaster knows the (approximate) location of the user and sends a location specific message. Thus, user location privacy is compromised. (ii) The broadcaster sends a message to all users, but the app displays only the messages relevant to the user's current location. The second approach is typically used when messages are intermittent. KatWarn, a German government crisis app that, once downloaded and granted access to location data, notifies users within a defined area of any major event that may impact their safety such as a natural disaster or terrorist attack. User privacy is compromised by apps using the first mode as the broadcasting agent receives information about the user's location. Apps using the second mode do not have this same limitation as location data is not reported back to the broadcaster.
In addition to the risk to the user's privacy with selective broadcasting, the same risks of identification of the diagnosed carrier and harassment of locations associated with the diagnosed carrier seen with broadcasting apply.
Further, requiring a user to sign up and subscribe risks decreased participation by possible users.
Unicasting informs only those users who have been in close contact with a diagnosed carrier. Unicasting requires government access data, not only of diagnosed carriers, but also of every citizen who may have crossed their path. The transmission is unique to every user. China developed a unicasting system which shows who poses a risk of contagion.
While highly effective at identifying users exposed to contagion for containment interventions, unicasting presents a grave risk for a surveillance state and government abuse.
In participatory sharing, diagnosed carriers voluntarily share their location trails with the public without prompting by a central entity, such as a government. Advantageously, with participatory sharing, diagnosed carriers retain control of their data and presumably consent to its release. Users are required to independently seek the information and assess their own exposure risks. However, these solutions present challenges as it is difficult to check for fraud and abuse. 
Risks exist for both the individual and the public with use of contact-tracing technology. The primary challenge for these technologies, as evident from their deployment in the COVID-19 crisis, remains securing the privacy of individuals, diagnosed carriers of a pathogen, and local businesses visited by diagnosed carriers, while still informing users of potential contacts. Additionally, contact-tracing technologies offer opportunities for bad actors to create fear, spread panic, perpetrate fraud, spread misinformation, or establish a surveillance state.
All containment strategies require analysis of diagnosed carrier location trails in order to identify other individuals at risk for infection. Diagnosed carriers, therefore, are at the greatest risk of their privacy being violated, for example, by public identification. Even when personal information is not published, these individuals may be identified by the limited set of location data points released. When identified publicly, diagnosed carriers often face harsh social stigma and persecution. In one example, data sent out by the South Korean government to inform residents about the movements of those recently diagnosed with Covid-19 sparked speculations about individuals' personal lives, from rumors of plastic surgery to infidelity and prostitution. Online witch hunts aiming to identify diagnosed carriers create an atmosphere of fear. As painfully articulated by the following quote, social stigma can be worse than the disease.
With all currently available contact-tracing technologies, the risk for public identification of the diagnosed carrier remains high. Further innovation is necessary to protect high risk populations.
Users also face privacy violations. Providing an exposure risk assessment to the user requires the user's location data in order to establish where the user's path has crossed with that of a diagnosed carrier. However, enabling access to 
Contact-tracing technology may, at times, violate the privacy of a non-user. Users and non-users are networked together through social relationships and environmental proximity. When a family member or friend's identity as a diagnosed carrier is revealed, non-users close to the diagnosed carrier may endure the same public stigmatization and social repercussions. When a business loses customers or faces harassment due to association with a diagnosed carrier's location trail, its patrons and, particularly, its employees bear the economic and social burden whether or not they are a user of contact-tracing technology. Non-users may be further negatively affected if location trails pinpoint sensitive locations, such as military bases and secure research laboratories.
Obtaining consent for any form of data collection and use helps manage privacy risks. Consent's utility in real-world settings, however, is often undermined. Language which is incomprehensible for typical users and a lack of real choice (e.g. users must often relinquish privacy and share their data in order to receive a service or opt not to use the service at all) severely limit the power of consent. Contact-tracing technologies have yet to overcome the challenges associated with obtaining true consent from the user. Typically, a user may be required to share their location with a third party in order to receive an exposure risk assessment.
During an epidemic, complex and quickly evolving data must be accurately conveyed to and understood by the entire public, including individuals with low health literacy. Serious harm, including heightened alarm among the public, may result from failure to appropriately communicate health risks. Contact-tracing technologies have potential to introduce misinformation and cause panic. For example, if users receive an alert about a possible contact location without appropriate information and understanding of the exposure time frame, some users will inaccurately conclude they are at high risk. Even when information regarding both location and time is provided to users, if the magnitude of the risk cannot be easily comprehended, an atmosphere of fear or a run on the medical system may be provoked.
Feeling a false sense of safety at having not received a notification of exposure, some users may underestimate their risk for disease. Users who no longer perceive a significant risk may be less likely to engage in other forms of disease prevention, such as social distancing. A false sense of safety may occur when the limitations of contact-tracing technology within a community are not clearly communicated to the public.
Technological interventions in human crises are often targeted for fraud and abuse. In South Korea, fraudsters quickly began blackmailing local merchants and demanding ransoms to not (falsely) report themselves as sick and having visited the business. Additionally, bad actors may force individuals to provide their location data for purposes other than disease containment, such as for immigration or police purposes. Fear of such abuse may prevent a contact-tracing system meant to help save lives from being adopted.
Hacking lingers as a serious risk for all data-gathering technologies with sensitive information, like health status and location. Hackers have successfully infiltrated apps and services collecting sensitive information before, with 92 million accounts from the genealogy and DNA testing service MyHeritage hacked in 2017. Data security must lie at the center of every effort to use location data for contact-tracing and containment.
Ensuring equity and social justice challenges many technologies, including contact-tracing. If participation requires ownership of a smartphone, some people, often those most vulnerable, the elderly, the homeless, and those living in lower-income countries, will not be able to access the technology. A lack of access to devices among vulnerable populations will remain a significant challenge for contact-tracing technology in the near future. Avoidance by the public may impact any business identified on a diagnosed carrier's location trail, but reduced hours or job loss hurt lower-income service workers most. Finally, abuse of data collection and violations of user privacy are inflicted more often upon those who are already most vulnerable to government surveillance.
In the following table, the various contact-tracing technological approaches are mapped against the reviewed risks and challenges.
The inverse relationship between accuracy of the provided risk assessment and user privacy for contact tracing technologies necessitates compromise by the user community. The core trade-off between utility and user privacy, diagrammed below, illustrates this and highlights the potential of Private Kit: Safe Paths to fundamentally alter this relationship.
Deploying any form contact-tracing technology requires contemplation of several risks outlined in the prior analysis.
Mitigation of these risks depends on thoughtful consideration of the trade-offs inherent to contacttracing technology and containment strategies. In the following, we review decisions required for these trade-offs and best approaches for risk mitigation.
Data must be collected from diagnosed carriers to facilitate containment of an epidemic. However, both data collection and release of that information to identified contacts may violate the diagnosed carrier's privacy. As the most vulnerable stakeholder in the containment strategy, several efforts must be undertaken to protect the diagnosed carrier's privacy to the highest degree possible. Limiting the publicly published data helps protect the known carrier's identity from the public. To date, with the exception of participatory sharing models, the diagnosed carrier's data must be shared with a third-party entity, requiring the carrier to relinquish at least some control over their data. Ending the need for third party involvement would represent an immense step forward in privacy protection for diagnosed carriers. Access and usage of the data by an entity, mostly governments, should be limited and highly regulated. Harsh penalties for the abuse of such data should be established. Obtaining true user consent further protects diagnosed carriers. Not all approaches in use today require consent to share personal data. Particularly in non-democratic regimes, diagnosed carriers may be unable to deny consent. In other instances, all users must consent to share their data in order to be informed of their own exposure risk. We believe no one should be obligated to share their personal information. Time limited storage of location trails further protects the privacy of diagnosed carriers. Finally, using an open-source approach to create an app fosters trust in the app's privacy protection capabilities, as independent experts and media can access and evaluate the source code.
Containment of an epidemic requires publication of sites of known exposure to a diagnosed carrier to the public. Yet doing so risks harassment of local businesses at these sites. Providing broader location data may better protect the privacy of a local business, but also affects the accuracy of the risk assessment. Broad location data, such as notice of a 100x100m area into which a diagnosed carrier sojourned, may still identify a business. Any contact-tracing approach must balance the public health benefit of disease containment against the threat of economic hardship for local busi-nesses connected to the epidemic.
There is no easy answer to this trade-off as any choice impacts utility of the technology and risks affecting the viability of the business. Evaluating the risk versus benefit of location data release should occur on a case-by-case basis. The time frame of possible contagion must be released so the users may understand the limits of the exposure risk. Critically, the entity publishing the location data should consult with the local business and inform the business of any decision before the public is notified.
Issues of access and inclusion are not easily resolved by contract-tracing technology. Limited access to a device capable of utilizing contact-tracing technology and difficulty understanding and acting on the provided risk assessment overly affect the more marginalized of our societies. However, containing an epidemic outbreak quickly benefits everyone within a community. Implementation of contact-tracing technology within a community, even with unequal access, may increase the safety of all. The development of a simple GPS device that can share location trails may be a medium-term solution to some accessibility concerns, particularly in countries with limited smartphone penetration. Additionally, some form of access to information about a possible contagion must be made available to those without a smartphone and all information should be presented in a way that accounts for variation in health literacy among users.
The spread of misinformation cultivates instability and uncertainty during a crisis. Release of information on the spread of a pathogen to the public invites public speculation and fear-mongering and manipulation by bad actors. A false sense of safety for users may increase alongside increased efficiency of contact-tracing technology. Entities providing contact-tracing technology are also at risk to introduce error within the release information, despite best intentions. At this time, no strategies exist to eliminate these risks; however, such risks can be mitigated through educational outreach efforts and engagement with key stakeholders.
Storage of sensitive information invites attack by hackers. Trade-offs must be made in order to mitigate this risk. Only anonymized, redacted, and aggregated sensitive information should be stored. Use of a distributed network, rather than a central server, makes hacking less attractive, but requires providing security to multiple sites. In the long term, the safest way to store location data will be in an encrypted database inaccessible to all, including the government.
Time limitations on data storage also work well to secure information and should be implemented in contact-tracing technology. During an epidemic outbreak, the appropriate amount of time for data storage equals the time during which a diagnosed carrier could have possibly infected another individual. For COVID-19, this time frame is set to be 14 to 37 days. Deleting data after such a short period, particularly during an outbreak of a poorly understood pathogen has risks. However, we feel this trade-off should be made for data security and user privacy.
Our ability to accurately trace contacts of individuals diagnosed with a pathogen and notify others who may have been exposed has never been greater. Real risks exist, though, thus care must be addressed in the design of the solution to prevent abuse and mass surveillance. As a beginning to the discussion of how to develop and deploy contacttracing technologies in a manner which best protects the privacy and data security of its users, we have reviewed varioustechnological methods for contact-tracing and have discussed the risks to both individuals and societies. Pri-vateKit: Safe Paths eliminates the risk of government surveillance. It draws on the advantages from several models of contact-tracing technology while better mitigating the challenges posed by use of such technology. We have presented a discussion of precautions which should be taken and trade-offs which will need to be made. We invite feedback and discussion on this whitepaper.