I don't know if they actually know whether that is the reason why covid-19 causes low oxygen levels. This article seems to be saying that it happens even when

I don't know if they actually know whether that is the reason why covid-19 causes low oxygen levels. This article seems to be saying that it happens even when pneumonia isn't present: https://www.sciencemag.org/news/2020/04/why-don-t-some-coronavirus-patients-sense-their-alarmingly-low-oxygen-levels https://www.sciencemag.org/news/2020/04/why-don-t-some-coron... Unfortunately, the article is not as clear as I would hope, but that's how I read it at least.