Inside Fitbit’s Plan to Detect Covid Symptoms With a New Wearable

We talked to the team behind the new Fitbit Sense, a $329 smartwatch dedicated to tracking your health

Illustration: Sandro Rybak

The main reason the coronavirus has shut down society isn’t that it’s deadly; it’s that it’s invisible. If you could see the damn particles — as a neon-yellow cloud on someone’s breath, or a neon-yellow patch on a doorknob — you could avoid it, and the disease wouldn’t spread.

Fitbit is in the business of making the invisible visible. From the vibration patterns of your footsteps, they reveal how active you are. From your arm-movement patterns, they determine what exercise you’re performing. From your heart rate and tossing-and-turning frequency, they can graph your sleep stages.

The company’s latest ambition is to expand that detection principle to your overall health. If a smartwatch could learn the patterns of an early Covid-19 infection, for example, it could save your life and thousands of others. Fitbit’s new smartwatch, the Fitbit Sense (announced today and out next month for $329) is the first step.

Like the company’s previous efforts, it’s not a do-everything cellular wonder toy like the Apple Watch. It doesn’t make phone calls or unlock your Mac. Its app store has a few hundred apps, not tens of thousands.

Fitbit’s newest smartwatch is heavily slanted toward biometric measurements. Credit: Fitbit

Instead, it’s primarily dedicated to health and fitness. The Sense is bristling with sensors. They cover one third of its entire surface and include:

Skin thermometer. The entire stainless-steel back, set against your wrist, is part of the new skin-temperature system.

Electrodermal activity (EDA) sensor. This sensor measures your physical stress level.

When you want an EDA measurement, you rest your palm on the watch’s face. Tiny electrodes pass a faint current through your fingers, measuring their impedance.

“As you get stressed out, or upset, or excited, you get these little microsweats,” says Shelten Yuen, Fitbit’s head of research. “Well, sweat is conductive. The more sweat, the more salty conductive stuff you have on your skin, the easier it conducts, and the lower the impedance.”

For now, the phone app uses this data primarily during de-stressing activities, like meditation, relaxation breathing, or mindfulness. “We give you feedback on how that session is going,” says Larry Yang, vice president of product.

Electrocardiogram (ECG). This time, the electrodes on the bezel team up with the plate against your wrist to complete a circuit through your body. The goal is to sense the voltage around your heart, and thereby display, on the phone app, a full-blown ECG graph. Doctors use ECG readouts to detect heart conditions like abnormal rhythms, heart attack, and blocked arteries.

PPG heart-rate sensors. Most high-end fitness watches use photoplethysmography to detect your blood flow: rapidly flashing LEDs against your skin, coupled with optical sensors that measure the reflected light.

The Sense’s new PPG system, which Fitbit calls PurePulse 2.0, incorporates six LEDs instead of one, as in previous Fitbits, which dramatically enhances workout accuracy. That way, says Yang, “If the watch is a little off-center, like when you’re running, and it’s not getting a clean signal, we can error-correct” to get a more accurate reading. (To save battery power, the extra LEDs light up only when you’re active or exercising.)

The heart sensor also measures the changes in your heart rate — pulse variability — which plays a key part in tracking your stages of sleep, your cardio zones, your daily active minutes, and your breathing rate.

SPO2 sensor. Those LEDs flash green to measure your pulse. But by flashing infrared light instead, they can detect your relative SPO2 (saturation of peripheral oxygen), which is how much oxygen your red blood cells are carrying. Breathing problems like sleep apnea and COPD can produce spikes in your oxygen saturation levels.

Accelerometer. Here’s the most ancient element, the great-grandchild of the very first Fitbit pedometer: It’s a three-axis motion sensor, which assists with step counting, exercise tracking, and sleep measurements.

Gyroscope. This six-axis sensor measures the angular velocity of your wrist. Today, Fitbit uses it only to track strokes and laps when you’re swimming.

Barometric altimeter. Why on earth would a watch need an altimeter? To track how many flights of stairs you take each day, of course.

Unless you’re a quantified-self nerd, your first reaction to all of this may be: Who cares? (“Morning, honey! My SPO2 was 96% last night!”)

The data’s value skyrockets, though, when it comes to disease detection — a new realm for wearables. The individual sensors on the Sense watch are not as accurate as hospital gear, and Fitbit is cautious not to suggest otherwise. “We want to avoid sending lots and lots of people into health care facilities unnecessarily,” Jonah Becker, vice president of design, says. But drawing on the petabytes of real-world data it’s collected from previous devices can help Fitbit make some interesting connections.

In May, Fitbit opened an opt-in Covid study of 100,000 Fitbit wearers. By the end of the study, about 1,000 of them had tested positive for Covid-19.

Suddenly, the company’s researchers had a remarkable opportunity: They could look backward through those people’s data to see if, buried in the biometric data, there were any early signs that indicated a Covid infection.

The answer was yes.

According to the Fitbit study, a Covid infection drives your breathing rate up, your resting heart rate up, and your pulse variability down. About half the time, these metrics change a day or two before you feel any symptoms. (Fitbit has submitted the study to a medical journal — the company declined to say which — but you can read it ahead of time.)

Fitbit’s survey found that a Covid infection may produce predictable changes in your heart rate, heart-rate variability, and breathing rate. Credit: Fitbit

Fitbit’s smartwatches can already measure those telltale signs, and the Sense’s new skin thermometer can detect if you have a fever, too. All the company has to do now is develop the software to alert you about a possible infection, so that you can get a proper test and start isolating. Such a software update should, in principle, work on Fitbit’s existing Versa smartwatches, too.

The nasal-swab Covid-19 tests people get today “are not perfect,” says Becker. “The sensitivity is not fantastic. By the time you get tested, you are likely to have symptoms already. So if there are ways that we can use our biometrics to help people identify an infection sooner, there’s a lot of value in that.” Remember: The Fitbit could theoretically alert you a day or two before you start feeling sick. At that point, you could self-isolate immediately rather than, say, running your errands and possibly exposing other people to the virus.

The company says that it’s working on this feature, but has no release date in mind; developing machine-learning algorithms that can reliably determine patterns and make predictions, based on massive streams of sensor data from thousands of people, takes time to perfect. Yuen notes, for example, that training the Fitbit’s software to pick up subtle changes in your heart rate took years, but that it can now find telltale heart-rate variations that humans might miss. “I’ll be staring at this data and sometimes I just can’t see it,” he says. “But machine learning can find these very small heart-rate signals that we can’t even see with our own eyes.”

The fact that the company is working on Covid tracking — that its watches already have all the biological measurements they need — leads to another question: What other invisible conditions could these sensors detect early, when there’s still time to intervene?

“I feel like the question might be, what couldn’t it do?” says Yuen. “The treasure trove of this data — it’s not clear yet what the limits are.”

For now, the Sense’s attempts to alert you of impending medical issues are limited. You can opt to get notifications if your heart rate goes over or under specified thresholds, which could indicate bradycardia (heart beating too slowly) or tachycardia (too fast), both of which require medical intervention. A software update will soon offer a similar feature for SPO2 readings.

And the Sense, like the Apple Watch before it, can already detect heart-rhythm irregularities that suggest atrial fibrillation. That’s the world’s most common heart-rhythm abnormality, affecting over 5 million Americans.

When you have a-fib, the top chambers of the heart (the atria) occasionally quiver instead of pumping. Your blood doesn’t move, and it clots. In 35% of a-fib patients, these clots eventually break off and float to the brain, causing a stroke. The clots can also cause heart attack, kidney disease, and dementia. Clearly, early warning that you have a-fib might be very helpful.

“The treasure trove of this data — it’s not clear yet what the limits are.”

Doctors can test you for atrial fibrillation in their offices — but if you’re not having an a-fib attack at the time of the test, they can’t catch it. That’s why having a heart monitor on your wrist all the time is a big deal, especially if you’re at risk of a-fib or have some history of it.

The Sense’s ECG test involves laying your fingers on the bezel for 30 seconds. If the watch discovers suspicious atrial activity, it advises you to get checked out by a doctor. “We also point you to help articles on how to interpret the data, and what next steps you should take,” says Yang.

High blood pressure detection should one day be possible, too. Researchers have found a correlation between pulse arrival timehow fast the blood moves in your cardiovascular system — and hypertension.

To measure pulse arrival time, you’d need two sensors: an ECG to know when the heart is pumping, and PPG to detect each pulse’s arrival in your extremities. The Sense watch has both. “We’ve been looking into it,” says Yuen.

And what about Type 2 diabetes? It’s a largely preventable disease that affects about 32 million Americans, can lead to heart disease, blindness, and kidney disease. One of its symptoms is decreased circulation in your feet — and in your hands, where a smartwatch might notice the change.

There’s a microphone on these watches, too. At the moment, Fitbit’s not using it for biometric purposes — but why not? It could hear you coughing. It could hear you snoring. (Fitbit is, in fact, testing a snore-detection feature.)

The Apple Watch lets you know when the sound levels around you have gone over 90 decibels for three minutes. But what about chronic noise levels?

“Studies show that high levels of ambient noise can lead to high levels of stress, which can affect things like blood pressure,” Yuen says. “A lot of people live close to train tracks and highways. A humble thing like a microphone might help us understand your relationship to your environment, which then relates to your stress and your cardiovascular health.”

Health (data) risks

Fitbit knows a lot about you and your life — well beyond what its watches measure on your wrist. Because you fill out a profile on its phone app, and because Fitbit’s watches either contain their own GPS tracker or use the one in your phone, Fitbit knows your age, gender, height, and weight; where you live and where you go; and the entire history of your sleeping patterns, heartbeats, and activity. If you log what you eat, it even knows your nutritional background.

On one hand, Fitbit’s corporate pivot into disease detection means that protecting all of this data is even more essential. The company has, in the past, been the victim of data breaches that exposed usernames and passwords, potentially allowing bad actors to log into user accounts to view health information — and in Fitbit’s disease-detecting future, the leaked data could include your Covid-19 status. It would help if Fitbit offered multi-factor authentication as a native feature (the company says that’s coming soon).

For now, Fitbit notes that it encrypts all data between your device and the mothership; is entirely within your control to collect or delete; and is aggregated and anonymized when the company uses it for research. A Covid infection is also already what the government calls a reportable disease: You yourself are legally required to report it to your local health authority. And, of course, using Fitbit’s Covid-detection feature would be optional.

But as we now know, it’s not always enough for a technology company to invent a feature; it must also consider the unanticipated consequences. Some experts predict that corporations, schools, and stores could require that their employees, students, and customers use Covid contact-tracing software as a condition of entering the building, as a kind of “immunity passport.” (Already, you can’t enter an Apple Store without getting a temperature check.) And what would this data mean to the governments that already use contact-tracing data in ways that would horrify Americans?

On the other hand, having so much biological, geographical, and demographic information about you could someday permit Fitbit to make sophisticated, specific, truly valuable observations about you, like these:

  • “The pollen count is very high where you are, which could trigger your asthma. Best to do your workout indoors today.”
  • “When you have more than one glass of wine at dinner, you wake up more often during that night’s sleep.”
  • “On mornings when you run, you eat 22% fewer calories and fall asleep 11 minutes sooner.”
  • “Your stress is elevated 45% every time you travel to Cincinnati, where your parents live.”

Fitbit hasn’t promised any of these features — but all of it is possible, and for Yuen, it represents the ultimate goal line.

“I liken Fitbit, in its end state, as a mix between a loved one who cares about you, and your personal coach/companion,” he says. “For example, my dad can look at me and say, ‘You look like you’re getting sick,’ or, ‘It seems like you’re upset.’ He’ll notice these things well before I do — or perhaps because I’m in denial. With the wealth of data in these wearables, we may be able to get to a similar spot.”

In other words, the hardware to make a new realm of invisible things visible is already here; we just need to parse the ocean of biometric data that it produces. “There’s a whole world that’s open to us,” Yuen says.

Update: A previous version of this article incorrectly described two of the Sense’s features. It does contain a speaker, and to activate an EDA measurement, the user rests their palm on the watch’s face.

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