Fitbit data traces Covid's assault on the body

It's not good, folks

Truly sorry for the aggressively hairy wrist in this photo. Unfortunately I couldn’t find any on-topic shots of smooth-wristed people. (Getty Images)

It takes nearly three months, on average, for the heart of a typical Covid patient to return to normal function after contracting the disease.

That’s one of the key findings of a clever new NIH-funded study that uses data from Fitbit and other smartwatches to examine how Covid alters the physiology of the people who become ill with it.

The study enrolled over 37,000 participants and their fitness trackers between March 2020 and January 2021. During that time, 875 individuals reported symptoms of a respiratory illness and got a Covid test. 234 of them tested positive for the disease, while the remainder presumably had other, less serious illnesses like the common cold.

That set-up let the researchers answer an interesting question: what does the trajectory of a Covid infection look like from the standpoint of the data collected by Fitbits and Apple Watches, and how does it differ from a run-of-the-mill respiratory infection?

Take a look.

This chart, from the study, plots the average change in resting heart rate among the Covid and non-Covid patients for more than four months following symptom onset. It’s immediately apparent that from a cardiovascular standpoint, a Covid infection is a wild ride: patients’ heart rates spiked immediately following symptom onset, then briefly plummeted, and then careened upward again and remained at an elevated level for months.

The heart rates among those with regular respiratory illnesses, on the other hand, showed virtually no change from normal.


More concerning, a subset of Covid patients experienced even more drastic cardio effects: roughly 14 percent “maintained an RHR [resting heart rate] more than 5 beats per minute greater than their baseline RHR that did not return to their normal for more than 133 days.” That, folks, is what long Covid looks like.

The exhaustion reported by many Covid survivors also showed up in the data, with the patients’ monitors reporting elevated sleep durations that didn’t return to baseline for more than three weeks. Physical activity, as measured in daily step counts, correspondingly took a big dive following a Covid infection.

So what do these numbers mean? A couple extra heart beats per minute may not seem like a big deal, but there’s a reason your resting heart rate is a key diagnostic indicator: faster hearts are associated with lower physical fitness, higher blood pressure, heavier weight, and a greater risk of premature death.

In the case of Covid specifically, the researchers think elevated heart rate is a sign of inflammation or disruption to the part of the nervous system that regulates processes like breathing and pumping blood — important stuff, in other words, and a possible clue as to why the disease is so deadly.

But if nothing else the study vividly illustrates that the big mortality numbers we all know — 600,000 dead, etc. — only tell part of the Covid story. Many of those who survive their infections deal with serious complications for months afterward.

In conclusion: if you haven’t already, get your dang vaccine.

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