Fitbit Study Suggests Wearables Can Detect Covid-19 Before Symptoms Appear

I work with a group that does research using data collected with wearables, including Fitbit, so this is very interesting to me. The HRV value dropping at the same time that the pulse rate increases is an interesting signal that seems easy to pick out.

It is a little surprising that they show both Respiration and HRV which are not available over the Fitbit API. I wonder how much other data they collect that isn't made publicly available.