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Why Bluetooth apps are bad at discovering new cases of COVID-19

Why Bluetooth apps are bad at discovering new cases of COVID-19


False positives and rampant privacy issues suggest we need to make a new investment in hiring people

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Illustration by Grayson Blackmon and Alex Castro / The Verge

Yesterday, we talked about the things that need to happen before we can begin to slowly re-open our cities: hospital bed supply catches up fully with demand; testing catches up fully with demand; we develop programs for quarantining new cases and informing their contacts that they may have been exposed to the disease; and the number of cases declines for 14 consecutive days.

Today let’s zoom in (not Zoom in) on the third point: building systems to enforce quarantine and to trace the contacts of those who get infected. Both are areas where public health officials believe that technology can play a role. But I want to describe why that role might be more limited than you assume — and, according to the experts I’ve spoken with, much less important than staffing up public health agencies to do the primary work.

First let’s talk about quarantine enforcement — making sure that people the state has ordered to stay at home are actually doing so. This is an area where technology can play — and is playing — a huge role. I’ve mentioned Taiwan’s “electronic fence” in this column a few times already, but here’s a quick refresher from Reuters:

The system monitors phone signals to alert police and local officials if those in home quarantine move away from their address or turn off their phones. Jyan said authorities will contact or visit those who trigger an alert within 15 minutes.

The technology here is not particularly complicated. With the cooperation of a telecom company, you can tie a person’s phone to a single cellular tower. If the phone pings another tower, or shuts off, the public health authorities contact you. This approach is invasive, somewhat disturbing, and by all accounts quite effective. It’s not clear to me how a similar program could be implemented without new legislation giving telecom companies explicit permission to share this kind of data — my inbox is full of lawmakers (appropriately!) calling for safeguards and oversight on any such government surveillance. But if the recent stimulus packages are any indication, that also seems like legislation that could be written and passed very quickly.

Note that tech alone doesn’t solve the enforcement problem. You also need people calling patients whose phones appear to be moving or have been shut off. You need people doing spot checks to make sure the person under quarantine hasn’t simply left their phone at the house and gone to church. And you probably need a place to house quarantined people that is not with their families, which are the most likely places for the coronavirus to spread. Tech is necessary, in other words, but not sufficient.

Now let’s talk about what might be the most challenging piece in the entire stack: contact tracing. Public health experts tell me that getting in touch with people who may have been exposed to a known COVID-19 case is one of the most important steps we’ll need to take to contain future outbreaks. But the how of it is complicated. While we’ve seen a Cambrian explosion of contact tracing apps around the world, it remains unclear how good or effective any of them have been. And as US government officials consider asking big tech companies to consider working on contact tracing solutions — and I’m told that they have already made inquiries with Facebook — that’s worth keeping in mind.

To get a sense of how this played out in one country that has done a relatively good job containing the coronavirus outbreak, let’s turn to South Korea. Derek Thompson has a good piece on contact tracing in the Atlantic that describes how it has played out there. The country seemingly skipped over traditional contact tracing completely and went straight to putting new coronavirus victims on blast in the new public square — other people’s smartphones:

The government uses several sources, such as cellphone-location data, CCTV, and credit-card records, to broadly monitor citizens’ activity. When somebody tests positive, local governments can send out an alert, a bit like a flood warning, that reportedly includes the individual’s last name, sex, age, district of residence, and credit-card history, with a minute-to-minute record of their comings and goings from various local businesses. “In some districts, public information includes which rooms of a building the person was in, when they visited a toilet, and whether or not they wore a mask,” Mark Zastrow, a reporter for Nature, wrote. “Even overnight stays at ‘love motels’ have been noted.”

New cases in South Korea have declined about 90 percent in the past 40 days, an extraordinary achievement. But the amount of information in South Korea’s tracing alerts has turned some of its citizens into imperious armchair detectives, who scour the internet in an attempt to identify people who test positive and condemn them online. Choi Young-ae, the chair of South Korea’s Human Rights Commission, has said that this harassment has made some Koreans less willing to be tested.

So far, South Korea appears to be an outlier in this approach. Other countries are opting to build much more targeted interventions, using phones’ GPS and Bluetooth signals to passively track the proximity between individuals and inform potential contacts after someone gets infected. Singapore, which built an app called TraceTogether that monitors Bluetooth activity, “offers perhaps the most likely model for the West,” Thompson writes. The country is making TraceTogether available as an open-source project.

To the extent that they’ve been written about to date, these passive tracking apps are generally considered in terms of their privacy implications. Who collects the data? Where does it get shared? Can it be linked back to individual patients? How long should that information be stored?

Already, various academics and entrepreneurs are working on passive tracking apps that attempt to solve these issues. At Wired, Andy Greenberg reviews three such efforts, and all of them are absolute Rube Goldberg machines. Here’s one of the apps under development:

Covid-Watch uses Bluetooth as a kind of proximity detector. The app constantly pings out Bluetooth signals to nearby phones, looking for others that might be running the app within about two meters, or six and a half feet. If two phones spend 15 minutes in range of each other, the app considers them to have had a “contact event.” They each generate a unique random number for that event, record the numbers, and transmit them to each other.

If a Covid-Watch user later believes they’re infected with Covid-19, they can ask their health care provider for a unique confirmation code. (Covid-Watch would distribute those confirmation codes only to caregivers, to prevent spammers or faulty self-diagnoses from flooding the system with false positives.) When that confirmation code is entered, the app would upload all the contact event numbers from that phone to a server. The server would then send out those contact event numbers to every phone in the system, where the app would check if any of the codes matched their own log of contact events from the last two weeks. If any of the numbers match, the app alerts the user that they made contact with an infected person, and displays instructions or a video about getting tested or self-quarantining.

All of these efforts seem to skip over the question of whether a Bluetooth-reported “contact event” is an effective method of contact tracing to begin with. On Thursday I spoke with Dr. Farzad Mostashari, the former national coordinator for health information technology at the Department of Health and Human Services. (Today he’s the the CEO of Aledade, which makes management software for physicians.) Mostashari had recently posted a Twitter thread expressing skepticism over Bluetooth-based contact tracing, and I asked him to elaborate.

The first problem he described is getting a meaningful number of people to install the app and make sure it’s active as everyone makes their way through the world. Most countries have made app installation voluntary, and adoption has been low. In Singapore, Mostashari told me, adoption has been about 12 percent of the population. If the United States had similar adoption, you’ve now made your big contact-tracing bet on the likelihood that two people passing one another have both installed this app on your phone. The statistical likelihood of this is about 1.44 percent. (It could be higher in areas with greater population density or where the app was more widely installed.)

The second problem is that when these Bluetooth chips do pass in the night, you should expect a large number of false positives.

“If I am in the wide open, my Bluetooth and your Bluetooth might ping each other even if you’re much more than six feet away,” Mostashari said. “You could be through the wall from me in an apartment, and it could ping that we’re having a proximity event. You could be a on a different floor of the building and it could ping. You could be biking by me in the open air and it could ping.”

All of this seems really problematic even before you consider asking Apple or Google or Facebook to make a contact tracing app and promote it through their own channels. We’ve spent three and a half years having a discussion about the shortcomings of these companies when it comes to protecting our data privacy; putting them in a position to oversee a project as intimate and sensitive as disease infection seems like a bad idea. (My own sense in talking from executives at Google and Facebook in recent days is that they are eager to help with crisis response, and are already doing so in various ways, but basically have no interest in this particular part of the response.)

So that leaves us with two remaining questions: what should Big Tech do, and what should the government do?

Public health experts I’ve talked with have been enthusiastic about efforts from Facebook and Google to use aggregated, anonymized data to display movement patterns — an important measure of the effectiveness of stay-at-home orders. They like Apple’s COVID-19 screening tool and Facebook’s collaboration with Carnegie Mellon University to encourage users to self-report symptoms — to the university, not to Facebook. These projects won’t solve the crisis on their own, but they’re good and useful tools for giving public health officials something close to a real-time look at how the disease is spreading through communities. And if there are other tools that they can build — particularly ones that rely on aggregated and anonymized data, rather than personally identifiable information — I think companies should continue exploring them.

And what about the government? The good news is that our public health infrastructure already has a lot of practice with contact tracing, thanks to our dear old friend the sexually transmitted infection. Come down with HIV, chlamydia, or gonorrhea, and a good county health agency will work with you to contact anyone you may have exposed since becoming infected. (Here’s a good piece by Ryan Kost in the San Francisco Chronicle about how the city’s experience with HIV/AIDS in the 1980s led it to dramatically beef up its public health infrastructure, which contributed to its admirably quick response to the threat of COVID-19.)

Those same tactics — public health investigators making phone calls and working out in the community — seem to be the most effective tool we have for contact tracing. And the good news is that savvy public health departments — like San Francisco’s — are already ramping up. Here’s James Temple in the MIT Tech Review on what he describes as one of the first such efforts in the country:

The Department of Public Health is supplementing its own staff with city librarians and dozens of researchers, medical students, and others from the University of California, San Francisco. City health workers have already been contact tracing on a small level, but they plan to significantly scale up the effort over the next few weeks. The team includes about 40 people and could rise as high as 150.

The task force will interview every patient who tests positive and provide necessary support to ensure that all are completely isolating themselves, down to helping them find and get to shelter if necessary. They also expect to reach out to between three and five people that patients came into contact with in the preceding days. They’ll alert them they may have been exposed, ask them to limit their contacts, and either encourage them to go in for a test or bring one to them. Those who test positive will trigger additional rounds of interviews and contact tracing.

Experts I’ve spoken to say that there are software tools that could help public health workers: a website or app where people affected by COVID-19 can voluntarily upload their contacts, for example, making tracing easier for the health department. But you still need people to contact them.

Tom Frieden, a former director of the Centers for Disease Control and Prevention, told me a story to illustrate this point. Earlier in his career, when he was working on disease prevention with the New York City Department of Health and Mental Hygiene, his team was tracking a young man who suffered from drug-resistant tuberculosis. The man wound up in juvenile detention, but then escaped. He had no cell phone to trace. But Frieden dispatched a team to find him, and eventually succeeded and got him into treatment.

The point was that you cannot design a Bluetooth app that finds this man.

But you can hire people to find him. Frieden says we will need an extraordinary amount of people — as many as 300,000, he says. “You’re talking about something like a hundred-fold increase in public health capacity,” Frieden said. “A lot of it can be done by phone banking. But a lot of it involves going out there and knocking on doors.”

The “good” news is that there are lot of people who are recently out of work and may be considering new career opportunities. It seems like one of the best ways we could spend stimulus money is in helping state and county health programs expand their capacity to hire people for contact tracing.

And we can keep exploring new tech-driven solutions for contact tracing, too. But for now it seems worth saying that there’s little evidence that phones are good at contact tracing — and a lot of evidence that human beings are. As we prepare to begin re-opening society, the biggest investment we need to make is in people.

* * *

What did I miss? What did I leave out? I’m a bit out of my comfort zone here, so if I’ve made a mistake in my facts or logic please let me know so I can fix it and share with everyone in our next issue.

The Ratio

Today in news that could affect public perception of the big tech platforms.

⬆️ Trending up: TikTok pledged $375 million in support of COVID-19 relief efforts. The aid includes $250 million in funds, $100 million in ad credits, and $25 million in ad space for public health information.

⬆️ Trending up: Facebook is donating another $2 million to mental health resources amid fears that the pandemic will lead to increased depression and related issues.

⬇️ Trending down: Amazon workers at a fulfillment center in Riverside, California filed a complaint yesterday, asking the state to investigate what they say are dangerous working conditions that pose a threat to public health during the coronavirus pandemic.


How Zoom CEO Eric Yuan built a conferencing app that suddenly became the social network of the pandemic. “I never thought that overnight the whole world would be using Zoom,” he says. “Unfortunately, we did not prepare well, mentally and strategy-wise.” (Drake Bennet and Nico Grant / Bloomberg)

Elizabeth Warren sent a letter to Zoom CEO Eric Yuan demanding information about the company’s privacy policies, particularly as they relate to children. The letter details a number of Zoom’s recent security and privacy issues, including the recent “Zoombombing” harassment attacks. (Russell Brandom / The Verge)

Jeff Bezos made a surprise visit to an Amazon warehouse near Dallas, where employees are working hard to meet a surge in online orders as customers shelter at home. The move comes amid lingering concerns about worker safety at the company’s warehouses. (Spencer Soper / Bloomberg)

A manager at an Amazon warehouse in Pennsylvania told staff not to touch shipments from another Amazon facility for 24 hours, since that facility had seen a cluster of COVID-19 cases. The Occupational Safety and Health Administration is opening an investigation into working conditions at the warehouse. (Matt Day, Spencer Soper, and Josh Eidelson / Bloomberg)

Instacart is introducing a pair of new delivery options in an attempt to deal with increased demand due to COVID-19. Fast & Flexible and Order Ahead are both designed to increase the number of available delivery windows, depending on whether customers are willing to be flexible with delivery times or if they want to plan ahead. (Jon Porter / The Verge)

Yelp is cutting 1,000 jobs and furloughing another 1,100 employees amid a massive drop in business. It’s the latest company catering to small businesses that has seen much of its customer base decimated amid the COVID-19 outbreak and related shutdowns. (Ina Fried / Axios)

A simulation from researchers in Belgian showing runners and bikers put other people at risk by spreading droplets when they exhale, cough, or sneeze has gone viral. But the findings haven’t been published in a study (even one that’s not peer-reviewed). It’s another case of an armchair epidemiologist going viral on Medium with shoddy science. (Jason Koebler / Vice)

Universities are rolling out chatbots and virtual assistants whose speed and tone can simulate text conversation, as students continue to learn remotely. Some can help students navigate issues they might have learned about on a campus visit, like plans for orientation and choosing classes. (Laura Pappano / The New York Times)

Chinese companies are going to extreme lengths to stave off new outbreaks of the novel coronavirus as they reopen for business. They’re now required to supply employees with face masks and check everyone’s temperature daily. It’s a crucial test of whether a country can keep the infection curve flat after lifting social distancing. (Eva Dou / The Washington Post)

The Unicode Consortium, the group behind emoji releases, announced it won’t release new emoji next year. The good news is that the emoji that were announced earlier this year, like the olive, beaver, and plunger, will still be available this fall. (Ashley Carman / The Verge)

Microsoft thinks the coronavirus pandemic will change how we work and learn forever. The company just released a report about remote work habits, noting that demand for Microsoft Teams surged worldwide last month, jumping from 32 million daily active users to 44 million in just a week. (Tom Warren / The Verge)

Webcams have become impossible to find as people stay quarantined at home. Third-party resellers have seized upon the scarcity by marking up webcams at ludicrous prices. (Chris Welch / The Verge)

A surge in internet usage is straining our networks. But it’s also prompting a wage of updates and upgrades that are making the internet stronger than ever. (Will Douglas Heaven / MIT Technology Review)

The volume of phone calls has surged more than internet use as people want to hear each other’s voices in the pandemic. It’s a trend that has surprised even the biggest telecom providers. (Cecilia Kang / The New York Times)

Virus tracker

Total cases in the US: At least 449,260

Total deaths in the US: More than 16,000

Reported cases in California: 19,043

Reported cases in New York: 159,937

Reported cases in New Jersey: 51,027

Reported cases in Michigan: 20,220

Reported cases in Louisiana: 18,283

Data from The New York Times.


The politics of 2020 now look much less ominous for Big Tech. The combination of changing priorities due to the coronavirus pandemic, along with critics like Bernie Sanders and Elizabeth Warren dropping out of the presidential race, mean the techlash has largely faded away. Here’s Eric Newcomer at Bloomberg:

Biden’s attacks have never provoked the concerns as those from Sanders and Warren. He has deep ties to the tech industry; his former director of communications, Jay Carney, is now Amazon’s top spokesman. Biden has also repeatedly framed his administration as a continuation of the Obama years, and several former Obama officials have set up shop in Silicon Valley.

While the tech industry rank-and-file mostly donated to the industry’s antagonists, its executives seemed most excited about younger moderates Pete Buttigieg and Cory Booker. Biden is a happy consolation prize.

Antitrust regulators in France ordered Google to pay publishers to display snippets of their articles on the company’s news service. The regulators gave Google three months to work out a deal with publishers. (Gaspard Sebag / Bloomberg)


Facebook is testing out a new feature called “Campus” which can only be accessed by college students, according to expert app researcher Jane Manchun Wong. The feature requires a .edu email address. Once you’re in, you can fill out a profile with your graduation year, major, minor and dorm if you want to find friends. (Mariella Moon / Engadget)

Facebook is adding a “quiet mode” that mutes most notifications. This feels like a move from a time when we all cared about how much we were looking at our screens. (Nick Statt / The Verge)

Things to do

Stuff to occupy you online during the quarantine.

Watch Sheryl Sandberg interview Dr. Julianne Holt-Lunstad, who researches the health impacts of loneliness, about staying emotionally healthy while physically distancing.

Watch a new music video from Thao & The Get Down made entirely on Zoom. The Oakland-based band had to pivot their plans for a live music video shoot due to the coronavirus pandemic. (Dani Deahl / The Verge)

Watch a new Saturday Night Live this weekend. The show is returning with fresh comedy with everyone working remotely.

And finally...

Talk to us

Send us tips, comments, questions, and your tech solutions for contact tracing: and