The Apple Watch can’t take your temperature. Neither can a WearOS smartwatch or the Fitbit Versa. During a time when more people than ever are concerned about having a fever, it’s a shortcoming that seems particularly glaring.
It’s not your watch’s fault, however. Skin readings, particularly on the wrist, are less than ideal for monitoring body temperature due to stress, sweat, and other external oscillating factors. For now, any attempts to do so would be inaccurate at best. If you’re worried about whether a lingering cough could be caused by a coronavirus infection, you’ll need to track your temperature the good old fashioned way and then input your results manually in Apple Health or Google Fit for now. Then you can consult your physician if you see an upward trend.
But that doesn’t mean wearable devices are powerless to help in the fight against COVID-19. Just like your smartwatch’s heart-rate monitor can alert you to possible warning signs of atrial fibrillation or sleep apnea, it can also spot warning signs that might signal your body is fighting a flu-like infection—if you know where to look.
Monitor your heart rate
App maker Cardiogram pushed an update to its Apple Watch and Apple WearOS apps last week that adds a new stat that shows average beats per minute during sleep. As it turns out, tracking fluctuations in your heart rate during sleep could signal that your body is fighting a viral infection like COVID-19.
Cardiogram co-founder Johnson Hsieh discovered the correlation after tracking his BPM during a bout with the seasonal flu in January. He noticed that his normal sleeping heart rate was about 10 beats per minute higher while his body was fighting the virus and returned to normal as his sickness subsided. The higher BPM was also evident during other parts of the day, but sleep is where it’s easier to spot.
It’s due to vasodilation, which is a fancy medical term for the expansion of the blood vessels during inflammation. As blood vessels expand, signals are sent to your brain to increase your heart rate and provide additional blood supply to inflamed regions.
“A pretty clear signal in your heart rate when you have symptoms that would otherwise be measured exclusively by a thermometer,” said Harish Kilaru, head of product at Cardiogram. “When your body is fighting an infection, both your sleeping BPM and your resting BPM are higher.”
Since Cardiogram is still studying how BPM correlates to viruses and other illnesses, it won’t be part of its UCSF partnership studies users can opt into, nor will it actively alert you to these types of fluctuations in your sleeping BPM. As Kilaru explained, other factors (such as consuming alcohol) can also lead to an increase in resting BPM, so it’s not an exact science just yet. While Cardiogram still recommends taking your temperature and consulting your physician with any concerns, heart-rate monitoring is one more way to stay on top of possible coronavirus symptoms.
“I think there’s a lot of potentials here because we have seen a connection between these symptoms and your heart rate,” Kilaru said.
A recent Fitbit study reached a similar conclusion. While examining resting heart rate data from 200,000 participants who used a Fitbit wearable device for at least 60 days from 2016 to 2018, researchers discovered s distinct correlation between elevated resting heart rate and influenza-like illness rates across five states (California, Texas, New York, Illinois, and Pennsylvania) and found the data significantly improved flu predictions. Additionally, weekly changes in resting heart rate closed mirrored changes in influenza-like illness rates.
Look to oxygen levels
While the Cardiogram app isn’t available for Fitbit OS, the Fitbit watch will natively track your sleeping heart rate each night. And if you have a Charge 3 or Versa 2, you’ll get an even better handle on your COVID-19 exposure due to their relative SpO2 sensors that can track oxygen levels in your blood.
Like Cardiogram’s method, Fitbit’s SpO2 sensor will record your blood-oxygen levels as part of its Sleep Score. While you won’t get a number, each morning you’ll find an Estimated Oxygen Variations chart that will show your levels throughout the night, which can be invaluable for tracking COVID-19 symptoms.
A typical blood-oxygen saturation level is above 90 percent, but COVID-19 sufferers can develop severe hypoxia, which lowers the oxygen levels in your blood due to damage to the walls of the air sacs in their lungs. If you see major fluctuations, which will be clearly marked, you can call or visit your local healthcare provider for information about testing.
Learning for the future
Monitoring resting heart rate and SpO2 tracking can be excellent indicators of the flu, but they’re still not as good as an actual body temperature reading. While there are very few wearables that are able to accurately show body temperature right now, that’s probably going to change in a post-COVID-19 world.
Smart ring maker Oura has already developed a direct skin temperature system that may provide a solution for smartwatches. It still uses a direct skin temperature for readings, but instead of showing your actual temperature, it charts fluctuations based on the wearer’s average baseline temperature using a custom algorithm, similar to how Fitbit calculates your blood-oxygen levels. So a mild fever would read +2.2 degrees rather than 100.8 degrees Fahrenheit.
With AI and machine learning coupled with a negative thermal coefficient (NTC) thermistors, tomorrow’s smartwatches could deliver continuous body temperature readings and alert you to flu-like fluctuations, much like the Apple Watch and others already do for heart health. For example, Kinsa is already using its smart thermometer data to create a Health Weather Map in the U.S. as a way to visualize infections. Combined with the heart rate data and SpO2 readings and future body temperature sensors, it could give health providers an invaluable tool for battling future pandemics within smaller communities.
But even without the ability to track body temperature, your smartwatch can still be useful in monitoring your own coronavirus symptoms. “Just because your wearable devices aren’t taking your temperature regularly,” Kilaru said, “it still provides you a clear and continuous data point that you can use on a day-to-day basis.”