Could a smartwatch have prevented my Covid illness?

Now that I am feeling mostly better after my first-ever Covid infection, a thought occurred to me: What could I have done to prevent this infection? After all, it cost me a week’s worth of work from home, and I had to miss three clinical shifts in the ER. (I was not going to exposure patients to viral loads like I had, nor would my hospital ask me to.) Oh, and it sucks. I’m feeling better every day, but this has not been a fun virus to have.

Would a booster have done prevented my infection? Not likely, in my case. Boosters offer short-term protection against infection, so unless I’d happened to have gotten a booster in April or May, my circulating antibodies would not have been enough to stave it off.

Could masking or other behavioral changes have made a difference? Generally speaking, these things could have helped. In my case, I did not have any exposures during the window when I possibly could have been infected. And when I was indoors in public briefly, I threw on an N95 mask anyway. If I’d been infected a week or two before that, I could have blamed some risks I took, like performing with my chorus and going out to dinner after. As it happened, my exposures the week I was infected happened to be really low.

How about rapid testing people who came into my home? Yes, this would have likely prevented my infection. I’m almost entirely sure that I picked up Covid from my Dad upon his recent arrival for a visit. Back in 2021-2023, we used to rapid test people who entered our homes, especially people who had travelled to be with us. We would often have visitors test daily for a few days, just to make sure we didn’t have any travel-related spread. In hindsight, my Dad may have had a symptom or two in the days leading up to his travel, but nothing to write home about. Piecing all the clues together, he was probably contagious when he arrived here, and I was infected in short order. So, I’m pretty sure he would have been positive on a rapid test upon arrival, if not the next day. We’ve been able to stop spread in this house before, so I am confident that if I’d been as zealous with our trove of rapid tests as we used to be, I would have avoided this infection.

Could a smartwatch have helped? I don’t own one, nor does my Dad. So, in this case, no. But it occurs to me that the fact that we have become a bit more lackadaisical in our efforts to avoid Covid (to the point where I forgot to rapid test my parents when they arrived; albeit, in their case the people I’d be most worried about getting Covid would be one of them, now that my kids are all fully vaccinated) leaves room for a new tool that I might consider.

Smartwatches, it turns out, can provide advanced notice that something is amiss in our bodies. In fact, colleagues of mine recently published a study in a Lancet network journal that demonstrated the power that smartwatches may have in giving us notice that we may be sick.

Volunteers were given smartwatches, and they wore them for years. During that time, of the 4,700 participants, there were 490 confirmed cases of influenza, 2,206 Covid-19 illnesses, and 320 bouts of bacterial strep throat (Group A Strep).

What did the smartwatches reveal? Heart rate changes. The baseline heart rates of smartwatch wearers had statistically detectable increases well before testing for the three pathogens was sought out. In the case of flu, heart rates increased around 68 hours prior to when they actually got tested; for Covid, increased heart rates were detectable 64 hours prior to testing; for strep, the lead time was 58 hours. Two or three days of advanced notice is huge in disease transmission dynamics.

On top of that, the participants kept track of their symptoms. So from this, we can see that there’s a pretty predictable pattern:

1. Heart rate increases.

2. Symptoms develop (on average 2-3 days later).

3. Testing occurs (on average 1.5-2 days after symptoms).

So the interval between the smartwatch “canary in the coal mine” and the time that patients actually got tested was often 2-5 days. This is massive. (Some of this is cut down by the use of rapid tests, which, as you all know, I think are underutilized.)

Heart rate changes after exposure to flu (top), Covid (middle), and strep (bottom). Snir et al. Lancet Regional Health—Europe. Annotations by Inside Medicine.

As you can see in the middle panel (Covid), the first symptom came around 5 days after exposure, and a full day after smartwatches picked up something being amiss. Please realize that these intervals are exactly why these viruses are successful entities that we can’t get rid of (side note: I do think the interval between exposure and symptoms is often a lot shorter than 5 days, but these are averages).

A better way forward?

It looks to me that smartwatch data applied at the individual level has a real chance to lower the reproduction number of a pathogen—that is, to how many people the average infected person spreads a pathogen. In a world in which even someone like me can’t stay vigilant with rapid testing forever, perhaps a smartwatch could be the nudge I need to do a test. (There are commercially available apps that supposedly do this; I have not vetted them.) Instead of doing a rapid test when I think of it, I would do one when my watch alerts me that my biophysical baseline has changed unexpectedly. Of course, for my infection to have been prevented, my Dad would have had to have had a smartwatch to alert him that something in his body was off. Seems like a good birthday present for him.

Have you used a smartwatch to detect illness? What are your questions and ideas? Share them in the Comments!