New Apple Watches Can Predict When You’ll Get Sick, Users Say. Can a Health Tracker Really Do That?

Wearables can help you glean insight into behaviors like your daily steps and sleep patterns, but some people claim they can also signal when you’re about to get sick.

Social media in particular is rife with people posting about the predictive powers of their wearables. “The vitals app knew I was sick three days before I did,” said one Reddit user, referring to a new app included in the Apple Watch iOS 11 software rolled out in September. 

“Woke up this morning with some congestion, etc.,” another Redditor wrote. “My vitals app showed I had an outlier last night while sleeping, indicating I’m sick or getting sick.”

Users point to subtle changes in key health metrics, such as heart rate and wrist temperature, that could indicate that their bodies are starting to fight off an infection. 

But while the ability to know you’re getting sick in advance seems incredibly handy, does your Apple Watch—or any other wearable—really have the ability to foreshadow future illness? Here’s what experts had to say.

Wearables like Apple Watches, Fitbits, Oura Rings, WHOOP, Garmin, and other fitness trackers generally work by continuously monitoring your body’s signals and using machine learning algorithms to identify changes from your normal levels. 

Most of these devices rely on photoplethysmography (PPG), a non-invasive optical technique that measures blood flow by using light and sensors to detect changes in blood volume at the skin’s surface, Thomas Maddox, MD, a professor of medicine at the Washington University School of Medicine in St. Louis and the executive director of the Healthcare Innovation Lab at BJC HealthCare, told Health.

“PPG is used to monitor heart rate, blood pressure, oxygen saturation, and cardiac output,” Maddox said. “It can also be used to detect atrial fibrillation.” 

Some wearables also detect other metrics, such as steps, floors climbed, stride length, calories burned, and sleep quality.

Trackers like the Apple Vitals app don’t explicitly say people are sick or about to get sick. But they do collect and analyze metrics that are strongly linked to the autonomic nervous system, which plays a crucial role in regulating various functions in the body, including the immune response, Varun Mishra, PhD, assistant professor at Northeastern University and director of the university’s Ubiquitous Computing for Health and Well-being (UbiWell) Lab, told Health.

As such, any deviations from your baseline patterns could suggest that your body is dealing with stress—and, as Mishra explained, fighting off an infection could be a reason why.

Pay particular attention if you have a rise in temperature and drop in heart rate variability, which is the variation in time between heartbeats, Mishra advised. Additionally, “an elevated heart rate at rest or during sleep could indicate that your body is working harder to maintain homeostasis, which could be an early sign of illness,” he said.

But while wearables may suggest that something is off with your body, experts agree that the devices aren’t reliable enough to predict illness in a clinical sense. “The ability to predict illness is not yet foolproof,” Mishra said.

Wearables only offer a partial view of what’s happening in your body, which is why you should consider the data alongside other factors, Mishra noted. For example, if you recently traveled abroad and your wearable shows unusual changes, it could mean you might be developing an illness. On the other hand, if you’ve had a late night of partying and drinking, you might also see similar changes. “This is where the context is important,” he said.  

Plus, wearables can sometimes miss important changes in biometrics, added Maddox. This could happen if the device doesn’t have enough contact with the body, the sensors aren’t sensitive enough, or the programming isn’t advanced enough to track biometric shifts accurately. 

That said, some research shows wearables have successfully detected illnesses like COVID-19 and the flu before symptoms appear, suggesting they could play a role in early illness detection, Brinnae Bent, PhD, executive in residence in the Pratt School of Engineering at Duke University, told Health. 

A 2021 study by Bent and her colleagues found that people who wore E4 wristbands, which measure metrics like heart rate, skin temperature, and movement, were able to detect influenza 24 hours before people showed symptoms. Other studies have indicated wearables can identify COVID-19 cases earlier than traditional methods, even in asymptomatic or pre-symptomatic individuals.

Some research has even suggested that wearables could spot early signs of chronic conditions, such as cardiovascular problems or mental health issues, by tracking changes in your body’s signals.

Ultimately, whether a tracker can accurately hint at illness depends on several factors, including the type of illness or infection, the type of wearable device you’re using, the detection algorithms, and even the conditions in which the device is worn, Bent said.

If your wearable device shows that your vitals have deviated from the norm, it can help you make more informed health decisions. For example, Bent said you might choose to stay home, wear a mask in public, or seek medical care, especially if you’re at higher risk due to preexisting conditions.

Paying attention to these anomalies can also encourage better health management, according to Mishra. By making you more aware of how things like sleep, stress, or exercise impact your body, trackers can prompt you to alter behavior before symptoms become severe. In some cases, these devices can even help “prevent the spread of illness” by alerting you to early changes, allowing you to rest or avoid social contact before symptoms appear, Mishra added.

If your wearable detects significant changes from your baseline levels, Maddock recommends consulting a healthcare professional to confirm the findings and receive guidance on what to do next. While wearables provide useful insights, it’s always important to “verify the signal and determine the next best course of action.”