I've seen multiple reports of severe blood clotting and strokes in COVID patients, including those who are otherwise asymptomatic, including people under 50.[1] Given that there is a pandemic going around with similar symptoms, I don't think this is a huge leap (but again, I am not saying it's anywhere near certain either). I probably should have used this source in my original comment, I just grabbed the first link I had handy.

[1]: https://www.biospace.com/article/covid-19-increases-risk-of-...

[2]: https://news.weill.cornell.edu/news/2020/07/what-is-known-ab...

[3]: https://directorsblog.nih.gov/2020/06/11/searching-for-ways-...