Fitness trackers provide conclusions related to COVID-19


Dear Doctor: I had a bad case of COVID-19 last spring. It took a while, but I’m mostly OK now. The only thing is, the numbers on my Fitbit aren’t as good as they used to be, especially my heart rate. Is that because of COVID? How long until I’m back to normal?

Dear Reader: It didn’t take long for fitness trackers to evolve from their original purpose. They began as mechanical devices designed to collect basic information, like the number of steps taken and distance traveled throughout the day. Thanks to advances in tech, they quickly progressed to electronic data-collection systems. Depending on the specific device you’re using, a fitness tracker can keep tabs on variables like distance, speed, direction and duration of movement, and even changes in elevation of the terrain. Through skin contact with an array of sensors, these devices can deliver information about a person’s heart rate while they’re active and while they’re at rest, and specifics about their heart rhythms. Some are even equipped with sensors and software that the manufacturers say can flag potential heart issues.

A study we wrote about last year found that subtle changes in health data collected by fitness trackers were surprisingly reliable predictors that someone was about to come down with the flu. The same predictive ability proved true with people who became ill with COVID-19. And now, sensors and algorithms are offering up conclusions related to long-haul COVID. As many of you doubtless know, that’s the name for the array of ongoing symptoms that some people experience for weeks, and sometimes months, after they have recovered from the initial infection.

Elizabeth Ko, MD and Eve Glazier, MD

One of the authors of that flu study, along with other researchers in California and Michigan, has examined the fitness tracker data of 234 people who had tested positive for the coronavirus that causes COVID-19. The group they followed had been moderately to severely ill. For at least three months after an initial diagnosis, the researchers saw persistent physiological changes, including an elevated heart rate. And because fitness trackers reveal patterns of movement, the researchers were able to identify changes to the individuals’ behavior, as well. This included a decrease in distance traveled each day and an increase in time spent sleeping. Although similar changes were seen in a group of people who had non-COVID-19 respiratory illnesses, they resolved far more quickly.

COVID-19 has a wide range of effects. This is proving to be true of long-haul COVID as well. For people in the study who were ill with COVID-19, it took an average of 79 days for their resting heart rates to return to what they had been before they became sick. Individuals whose illness was due to something other than COVID-19 saw their heart rates returned to normal just four days after their illness ended. For 14% of the COVID-19 group, a heart rate that was five beats faster than prior to their illness persisted for more than four months. We wish we had a better answer, but there’s no way to predict how long the after-effects of COVID-19 will last.

(Send your questions to [email protected], or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA 90025. Owing to the volume of mail, personal replies cannot be provided.)